The Stanley Family

A kids'/YA book series by Zilpha Keatley Snyder, three-time Newbery Honor winner, and author of Below the Root. The Stanley Family series focuses on an eccentric family and the various mysteries they encounter. That's really the only connection between the four books in the series, as each book is practically in a different genre than the others.

Each book involves a different mystery or adventure, but the theme of the mystery/adventure is different from book to book, ranging from supernatural occurrences to a neighborhood theft.

The books are:

The Headless Cupid - A Newbery Honor winner. Supernatural and occult-obsessed Amanda joins the Stanley family, and tries to get the rest of the kids interested in the occult with her, as their teacher. They end up investigating a poltergeist that apparently lives in their house.
The Famous Stanley Kidnapping Case - While living in Italy for a year, Amanda's bragging about having a rich father (her biological father, not Mr. Stanley) leads to all the Stanley kids being taken prisoner and held for ransom for a long time.
Blair's Nightmare - Blair encounters an enormous dog in the middle of the night, but no-one believes him at first. When David encounters the dog for real, the kids end up adopting it, and trying to keep it safe. Meanwhile, there are escaped convicts on the loose.
Janie's Private Eyes - Dogs are disappearing from the neighborhood, and a Vietnamese family that moved in is falsely blamed for it. The Stanley kids set out to prove their good name, and Janie in particular won't stop investigating every lead that comes to mind, much to David's chagrin.

The family consists of:

Jeff - Stepfather to Amanda, father to the other kids. Pretty much the standard "dad" character who won't tolerate nonsense from the kids. He and Molly have some disagreements on that at one point.
Molly - Amanda's mother, and the stepmother of the other kids. She's an eccentric painter and generally laid back, and encourages creativity.
David - The former oldest of the kids until Amanda came in. He's a responsible older brother who looks after the younger kids and has to make sure they stay out of trouble, which is easier said than done. The third-person narration always follows him. Age 11 in first book, 13 in the last.
Amanda - Newly added to the family in the first book. She starts out very eccentric and into the occult in the first book, before losing interest by the second. What doesn't change is her condescending personality, which she applies not just to kids but even adult authority figures as well. She's also somewhat tomboyish. Age 12 in first book, 14 in the last.
Janie - A know-it-all showoff who is smart for her age. She asks a battery of questions whenever something piques her interest, and also loves to gives a battery of answers to questions no-one asked. She has no compulsion about doing her own thing and coming up with her own plans independent of the other characters. Age 6 in the first book, 8 in the last.
Esther - Also "Tesser", based on her own name for herself when she was too young to pronounce "Esther" correctly. Twin sister of Blair. Age 4 in the first book, 6 in the last.
Blair - Twin brother of Esther. A quiet kid who has premonitions from time to time, which turn out to be true. He's also good with animals. Age 4 in the first book, 6 in the last.

The author herself said in a foreword that she considers Blair ("especially Blair") and Janie to be her favorite characters.

It's hard to describe the Stanley series as a whole, because each book is totally different, though they all tend to involve at least some mystery elements. The first one is a Slice of Life story about the older stepsister getting the rest of the kids interested in her obsession with the occult, as their mentor. Then later, a supernatural mystery shows itself. The second book is a suspense story about a kidnapping, how the kids deal with the situation, and how they escape. The third story is pretty much about a giant dog and how the kids deal with it, and the mysteries surrounding the dog. And the fourth is a straightforward Kid Detective mystery.

What each story has in common is a mixture of everyday home life, personality, character humor, fun, and suspense. The characters actually change and grow over the course of the series, taking on new traits or even changing their interests as they develop new ones.

Tropes used in The Stanley Family include:

Appropriated Appellation - Tesser is so named because she used to mispronounce her real name, Esther.
Bad Bad Acting - When the kids try to fake a miracle to scare guilt into their kidnappers, David cringes at his younger siblings' acting abilities.
Bound and Gagged - Happens to a kidnapper, not the kidnappee!
The Bully - Pete Garvey and his friends, in the third book, Blair's Nightmare, make David's life miserable, until Pete starts to change his ways and show another side to his personality.
The Cavalry - Happens in police form in Janie's Private Eyes.
Comic Book Time - Nearly two decades separates the first book in the series from the fourth. But the dialog, pop culture references and character interests update to the time each book was written.
Engrish - Used intentionally in the second book, where Amanda receives a phony love letter purporting to be from her Australian crush, but written in broken English, as it was really written by an Italian.
Free-Range Children - The kids have a lot of freedom to explore the countryside they live near.
Genre Roulette - Each book is so different from the others, it's as if the characters are the only unifying force in the series.
Heel Face Turn - In the second book, The Famous Stanley Kidnapping Case, at one point, two of the kidnappers, who are teenagers, pull this and turn on their boss. Pete Garvey pulls this in the fourth book, Janie's Private Eyes, when he realizes that his friends, who are the dognappers, are putting David and his younger siblings' lives in danger.
Heel Faith Turn - Implied to be a possible part of the cause of the Heel Face Turn in the second book, though the exact reason is left open to interpretation. David figures their kidnappers might be Catholic, since he's in Italy, and tries to get the other kids to fake a miracle to try to get through to their conscience.
Hypocritical Heartwarming - Amanda makes it clear to Paul Garvey that only she can give David crap.
Kid Detective - Janie in the third and fourth book, does her share of snooping around as well as traditional detective work. David joins in during the fourth book as well, even interviewing people.
Love At First Punch - Pete Garvey develops this for Amanda after she physically punishes him for his treatment of David. It also results in him no longer bullying David.
The Meddling Kids Are Useless - Played surprisingly straight in Janie's Private Eyes. While the kids actually do find out who the dognappers are and get proof, it's someone else who calls the cops and solves the case, rescuing the kids (who are in danger) in the process.
Never Say "Die" - At least not in English. When David begins to think that their kidnappers might kill them all, in the second book, the words "die" or "kill" are never used in the narration. The Italian word for dead, "morto", is used when Amanda is being told to write a ransom note in English by the Italian-speaking captors, but Janie refuses to translate the word.
Police Are Useless - Subverted. They end up being quite useful.
Prolonged Prologue - Happens in The Famous Stanley Kidnapping Case. Much of the first 90 pages of the book are the author spending a lot of time showing us unimportant things about Italy, before the plot kicks in.
Scooby-Doo Hoax - Happens in one of the books. A very skeptical character even assumes that it is one well before it's revealed.
Snooping Little Kid - Janie, in the third and fourth books. David has to chase after her sometimes when her snooping gets out of control.
Something They Would Never Say - In The Famous Stanley Kidnapping Case, which is set when the Stanleys are vacationing in Italy, Amanda receives a fake love letter supposedly from an Australian she has a crush on. But David notes that the English in the love letter is broken, and not something her crush - or any native English speaker - would ever write.
Strange Girl - Amanda in the first book, The Headless Cupid. She dresses and acts very odd, and has a love of the occult. But she changes over the course of the series, as it turns out that her Strange Girl demeanor is just a phase.
Tagalong Kid - The younger siblings, in many cases. Though Janie later on becomes more directly involved, and her ideas, along with Blair's visions, genuinely help.
Teens Are Monsters - To an extent. Kidnapping, including kids as young as 4? (to be fair, their actual target was 13, but her siblings were there too) Dognapping? They're teens just in it for the money, working for adults. On the other hand, some of them are redeemable. Though there is the one jerk who nearly runs over a 6 year old in his hot rod.
Unfazed Everyman - David Stanley. He's the "normal" character, as well as the one the narration follows. Everyone else in his family has their own quirks.
Write Who You Know - David and Amanda are based on kids the author once knew. David was based on a fifth-grade boy who "had a very mature sense of fair play" and had to look after a bunch of younger siblings. Amanda was based on a girl who showed interest in the occult "to punish her parents." These are practically descriptions of David and Amanda themselves, especially Amanda in the first book.