A couple of young adult novels by Diana Wynne Jones.
In Deep Secret, The Koryfonic Empire sits in the center of the multiverse. That is because the multiverse itself is shaped as an infinity sign, with the Koryfonic headquarters on the center of it all, its empire ruling across several worlds with an iron fist. Its domains include both sides of the multiverse. One side is "Ayewards", in which magic is common practice and easy to do, while the other is "Naywards", the worlds in which magic is nothing more than superstition, if not completely unknown. Our Earth is currently on the Naywards side, and seems to have been sliding back for a while.
Keeping watch over the balance of magic, a group of persons of great power called the Magids have a great number of agents on each one of the worlds, Ayewards and Naywards alike. One of the Magids from Earth, Stan, passes away. His successor Rupert Venables needs to find a successor himself to replace his late mentor. Among them he finds a great prospect on the person of Maree Mallory; however, he is horrified to find that she doesn't seem to be ethical enough for the Magid job, and all other candidates are on different parts of the globe. Looking for a way to contact them all, he meddles with the fates of each one to get them all together on a Fantasy Convention at Hotel Babylon near where he lives, but they are not the only ones to show up.
Not being enough trouble, Rupert is also suddenly called to the Koryfonic Empire. A terrorist attack blows up the palace and kills the Emperor and most higher-ups, and odd customs make finding the heirs very hard, what with most of them already being dead and the others protected by a very secure system. Rupert gets his hands full with both jobs as he searches for the missing heirs and searches for a Magid successor, and things start getting out of hand quickly.
The first book has a lot of tense situations while still poking fun at its own nature and, of course, at fantasy conventions themselves. It also is a young adult novel and thus has no qualms on mentioning sex and featuring High Octane Nightmare Fuel.
The second book, The Merlin Conspiracy, deals with an Ayewards world in which the King of England and his court are the keepers of magic. Traveling with an appointed magician, called the Merlin, he strives to keep magic under check. However, during an important political meeting the current Merlin suddenly dies and things get complicated. One of the kids travelling with the court, Roddy, gets stranded and ends up uncovering untold magical power and a conspiracy that plans on taking over all magic. But who will believe her?
On Earth Nick Mallory, Maree's cousin, is searching for a way to travel across different worlds and unexpectedly finds one. It's not easy for him, however, as at first he thinks it is all a dream and his lack of regional customs puts him into deep trouble. While travelling he ends up making many enemies, and wandering around the worlds he ends up meeting with Roddy's uncle, a Magid, and figures that Roddy's world is in trouble. Now he has to help Roddy save her world while dealing with all the problems he seemed to collect on other worlds.
The second book is somewhat tamer than the first and can be enjoyed without having read the first, but some backstory may be missed. It is also less definitely aimed at young adults; this is part of the reason it was not advertised as a sequel.
- Bumbling Dad: Nick's. Despite being a famous horror writer, he absolutely does not understand his teenaged son.
- Canon Discontinuity: Sort of justified. The "Also by this author," "About the Author," and book blurbs seem to like to pretend that The Merlin Conspiracy is a stand-alone novel, completely ignoring Deep Secret. This, however, is a Really Bad Idea--because Merlin is very obviously written as a sequel (the references to Magids and Nick's home life make zero sense otherwise), and is much more confusing without having read Deep Secret first. See above.
- Immune to Fate: Magids, to an extent. They're removed from the fatelines or so Rupert Venables thinks once they're made Magids, but Them Up There can still mess with them.
- The Multiverse
- Save Both Worlds: More like save all worlds.
- What Do You Mean It's Not for Kids??/What Do You Mean It's for Kids??: This is possibly why blurb copy likes to try and distance Merlin from Secret. Despite being written by a prominent children's/young adults' author, Deep Secret was originally published under an adult label and has more adult themes than her other works. (It has a couple instances of rather shocking violence, and a few references to sex, though it's never explicit.) The Merlin Conspiracy is slightly more child-friendly, but not by much.
- Alliterative Family: Rupert's brother Will Venables has six daughters, all of whose names begin with the letter V.
- Beautiful All Along: A variant of sorts. Maree doesn't actually get any more physically attractive, apart from a haircut and change of clothes, and she was never a looker to begin with. However, due to both understanding her personality better and the fact that she's very nearly just come Back From the Dead, Rupert perceives her as more beautiful, and that's good enough for him.
- Belligerent Sexual Tension: Rupert and Maree. Both of them having Jerk with a Heart of Gold tendencies helps nothing.
- Big Beautiful Woman: Zinka. Maree describes her as "plumpish" (but then, Maree's not small herself), and Rupert refers at one point to her "lovely, ample form."
- Carnivore Confusion: Everyone wonders if the centaur Rob eats meat. (He does.)
- Chaos Architecture: The Hotel Babylon--explicitly so, because someone is toying with the node that the hotel is set on.
- Chessmaster: Nick's mom and Gram White.
- Cloudcuckoolander: Andrew, though there's a reason for that.
- Doing It for the Art: Discussed. Nick and Maree are both disappointed that Nick's father discuss writing in such a mechanical, bland way at the convention. They agree that, despite what he says, he must love writing, or else he wouldn't be able to imagine his Eldritch Abominations so dotingly. And he does--ironically, he's just bad at articulating it.
- Face Doodling: Pulled by Zinka on a poor slob left drunkenly passed out on the stairs. Rupert notes that the vines she's drawing will bring him under her power.
- Fan Convention: The main setting for Deep Secret.
- Geas: Rupert defeats one antagonist by putting him under a geas not to cast magic. If broken, he would die. He does.
- Good Bad Girl: Zinka rather gets around (and makes spending money while on Earth by selling her, uh, interesting artwork at conventions), but she's quite well-respected and generally considered a really good person by everyone who meets her.
- Half Human Hybrids: Rob the centaur, and he's not the only centaur of the sort.
- Happily Adopted: Nick and Maree. Neither of their "fathers" is actually related to them. Maree starts calling her stepfather her "As-it-were Dad" and still loves him immensely. Nick has some difficulty getting to know his stepfather for real, but in the end, he's still better than Janine.
- Has Two Mommies: There's a group of Recurring Extras in the book who consist of a trio of ambiguously-gendered people who all share duty taking care of a baby. Rupert and Maree both spend time wondering what their story is.
- Interspecies Romance: Rob is half centaur. Centaur/human interbreeding is described as being a very bad idea, though, with the babies often stillborn (always, if the human is the mother).
- Jerk with a Heart of Gold: Nick, Rupert, and Maree all have rather bitter exteriors hiding kind hearts. In all three of their cases, it's mostly a defense mechanism--Rupert against his job, and Nick and Maree against their mother/aunt. Nick also has the excuse of being fourteen.
- King in the Mountain
- Love At First Sight: A variation occurs in Deep Secret. Rupert realizes, after meeting the real Maree for the first time, that he was deeply infatuated with her just from the vibe she gave off in the letter she wrote back to him. Repulsed by her appearance, he changes his mind but after spending more time with her, he realizes that he loves her anyway.
- Meet Cute: And lord, what a meeting it is...
- More Than Mind Control: In Deep Secret, Nick's mother uses this on him. (He's mostly immune by the time the story takes place).
- Must Have Caffeine: Nick is a classic example of this trope.
- No Celebrities Were Harmed: Neil Gaiman (or, at least, an unnamed famous author in a leather jacket whom Maree thinks is good-looking) makes an appearance in Deep Secret.
- In his memorial post about Diana Wynne Jones, Neil also says that Nick's Must Have Caffeine (and the sleep eating) were based on him.
- No Blood Ties: In centaur culture, it's noted that a male centaur is expected to be more loyal to his sister's children than his own. This is a plot point, as it explains Rob's upbringing: the (human) emperor was his father, and his uncle brought him up.
- Plucky Girl: Maree, to the point where even losing half her soul won't stop her.
- Rags to Royalty: Nick, Rob, and Maree are all royalty. Nick and Rob know; Maree doesn't, and when she finds out, she is NOT pleased.
- Rule 34: Actually invoked in-story, as the character of Zinka likes to sell her... interesting paintings at the convention.
- Screw Destiny: Rupert finds himself thinking this a lot.
- Selfless Wish: When Maree and Nick reach Babylon, Maree is supposed to wish for the other half of her soul back, but instead wishes for her dad to be cured of cancer, so Nick, in turn, has to use his wish on her.
- Small Name, Big Ego: An in-universe example in Mervin Thurless.
- Spirit Advisor: Stan.
- Ugly Guy, Hot Wife: Inverted with Maree and Rupert. Rupert is repulsed by Maree at first, her being chubby, a bit smudgy, and her voice being rather whiny. Of course, by the time she comes back from Babylon, he's convinced there's no more beautiful woman in all the Multiverse and wouldn't dream of having anyone else.
- Would Hurt a Child: Janine would murder several.
- You Are in Command Now: Davros, a mid-ranking military official, is promoted to emperor when the real Emperor, and almost all of his high-ranking staff, are assassinated.
- Adults Are Useless: Averted and played straight at the same time. Most of the adults that Nick and Roddy try to enlist the help of are very powerful and very clever. Unfortunately they either refuse to help because they don't believe Roddy or are kidnapped. Also, several adults are very useful before their kidnapping, but aren't brought back to Blest until after all the action has taken place, Maxwell Hyde being the prime example. Romanov is the primary aversion, as he's not kidnapped (although does get very, very sick and almost dies at one point), and spends the climax of the story undoing Joel's binding spells on the kidnapped off-screen.
- Ax Crazy: Older Japeth in The Merlin Conspiracy Though both his and Joel's younger selves weren't above murder.
- Balancing Death's Books: Roddy's grandfather, Gwyn, the lord of The Wild Hunt, finally gets his revenge.
- Berserk Button: Don't laugh at Japeth.
- Chessmaster: Joel and Japeth.
- Cold-Blooded Torture: What Japeth wants to do to Nick. Instead, he settles for human sacrifice.
- Crapsack World: Nick wanders through several. Loggia City is probably the most crapsacky world with living people in it that we see. At the climax, all worlds get varying degrees of this, with magic being permanantly altered and causing chaos, though it's implied they get better.
- Crystal Dragon Jesus: One of the universes Nick pops through in The Merlin Conspiracy had Loggia City, which is run by the Prayermaster and the Lawmasters.
- Enfant Terrible: The Izzies, in Merlin Conspiracy.
- Fainting Seer: In Merlin Conspiracy, when the new Merlin is introduced to give a prophecy, he begins weeping and faints. The audience is unimpressed, and complain that they wound up with one of the 'weepy' kinds of seer.
- The Fair Folk: The little people in The Merlin Conspiracy. They're friendly enough, but they don't look at all like traditional fairies (the way they're described makes them seem catlike in appearance, and with backwards-bending knees), and they carry off a (very unpleasant, admittedly) girl at the end.
- Knight Templar: In Merlin Conspiracy, the Prayermaster.
- Meaningful Name: In Merlin Conspiracy, Roddy's grandfather Gwyn.
- Offing the Offspring: In The Merlin Conspiracy: Sybil was going to offer Grundo up as the human sacrifice, until Japeth saw Nick.
- Self-Made Orphan: Joel and Japeth. (Although technically he was just their teacher/guardian).
- Stable Time Loop: In Merlin Conspiracy.
- The Stoic: Romanov
- Talking Animal: Mini, the elephant in The Merlin Conspiracy.
- Mini cannot actually speak; Nick just has a mental connection with her, and some other animals.