Pyrates

A four-book series that shares elements of pulp adventure, treasure hunt stories, and Stratemeyer-esque investigating/snooping, while attempting to update their tropes to fit modern times and be more plausible. Written by Chris Archer, the series consists of The Secret City, Eye of Eternity, Dead Man's Chest, and The Last Clue. All four were published in 2003.

On his 11th birthday, fifth-grader George van Gelder discovers that Captain Kidd, with whom he shares lineage, has a treasure map, and George happens to come across part of it. After recruiting a group of his friends, George sets out to see if they can find the rest of the treasure.

The group consists of:
 * George -- Protagonist and descendant of Captain Kidd. Looks up to Kidd, and considers him a law-abiding privateer rather than a villainous pirate. He can mimic other people's voices, a talent that freaks out his friends.
 * Derrick -- Has a Photographic Memory, which helps tremendously. He has "dark, curly hair", and is black according to the book covers. (No-one's race is given in the narrative) Gets grossed out easily.
 * Shannon -- A Tomboy who frequently dyes her hair, wears a choker, and dresses like a preteen rocker girl. She's very brave and sometimes cool under pressure, like when trying to divert George's dad's suspicions. She's said to be the only sixth-grader who's the leader of her own band, and one of the coolest kids in the school.
 * Renee -- A blonde girl who knows a lot about rock climbing and cave exploration, and is integral to the mission. Also a member of Shannon's band, and originally recruited by Shannon. Partly The Chick.
 * Paul -- Not technically part of the group, but a friend the kids make who helps out. A homeless boy who lives in the underground tunnels, has excellent night vision, and really knows his way around. He's generally distrustful by nature, but gradually becomes friendlier and opens up over time.

Along the way, they encounter many things they could have never imagined. A massive series of underground tunnels and caves under New York City, a small underground civilization in hiding, a dangerous and violent group of criminal rival treasure seekers led by the mysterious "Leroy", and booby traps placed by Captain Kidd himself centuries ago.

There's plenty of modern day adventure to be had, snooping to be done, and use of modern day knowledge to try to track down an ancient treasure that may or may not exist. There's also the mysterious Paul, who's homeless, knows his way around the dark tunnels but is easily hurt by bright lights, and is a hugely valuable asset to the kids, yet doesn't seem to appreciate their presence. Beyond that, there's also the issue of time -- the kids have to make sure they only leave when there's no-one home at George's house, and that they're back home on time. Or the times when they stay up very late at night, including literally into the next morning!

All in a day's work -- well, several weeks' work -- for this group of plucky 11-year-olds.

This book series contains examples of:

 * Adults Are Useless - Played straight for the first three books before being averted in the fourth book. George's dad actively gets involved in the adventure and helps out, although the kids do go on a few more adventures on their own. A certain event in the story pretty much forces George and his friends to let George's dad in on their secret. Another set of adults helps as well much later on.
 * Badass Bookworm - George's dad late in the fourth book.
 * Beethoven Was an Alien Spy - Captain Kidd has quite a backstory behind him, which ties into the events occurring in the present.
 * Beneath the Earth - There's actually a small civilization that lives underground, under the subways.
 * Big Bad - The mysterious "Leroy", who leads a gang of treasure seekers.
 * Big Damn Heroes - At one point in the third book, by George's friends as they come to help him.
 * Black Best Friend - Derrick. No-one's race is explicitly mentioned in the books, but his "dark, curly hair" is, and he's black on the covers, while fitting the book's description of his character.
 * Blackmail - George, using his ability to mimic voices to disguise himself as one of "Leroy's" henchmen, threatens "Leroy" with it.
 * Booby Trap - Captain Kidd's method of trying to prevent people from finding his treasure, naturally.
 * Bound and Gagged - Happens at various times to Paul, Shannon, George, George's dad's girlfriend, George's dad, and one of the bad guys.
 * Chekhov's Gun - Something that makes its first appearance in the first book, turns out be... a lot less harmless than it seems.
 * The Chick - Renee, somewhat.
 * Dismantled MacGuffin - The treasure map. The kids start with one piece, the bad guys have another, and there's more out there.
 * Exact Words - One of Kidd's notes mentions getting an "eternal reward" if certain instructions are followed. Shannon assumes it refers to the Eye of Eternity. It actually refers to death by one of his traps, which George realizes is pretty "eternal" indeed.
 * Famous Ancestor - Captain Kidd.
 * Flynning - Averted when swords and knives actually are used.
 * He's Got a Weapon! - "Leroy" changes the playing field by revealing that he has a gun, and considering his history of kidnapping and violence, he's not afraid to use it. When he loses his gun, he pulls out a knife, and he DOES use it.
 * Heroic Vow - George and his friends take one, and stick to it.
 * Hostage for Macguffin - The kids attempt this, with a fake version of the treasure. Later, it's attempted for real, with the actual treasure.
 * Insistent Terminology - George keeps insisting that Captain Kidd wasn't actually a pirate, but a privateer, thus making him not an outlaw. His friends give him some flak over it; after all, what licensed privateer loads up booby traps and hides treasure just like a pirate?
 * Kid Detective - In more of a "treasure seeker" sense than the usual "solves mysteries" sense. Since they do have to figure out clues and snoop around and investigate, they are still technically doing detective work.
 * Kid Hero - Of course.
 * Living With the Villain - The true identity of one of the bad guys...
 * MacGuffin - The Eye of Eternity, the treasure that they're looking for. And also, the pieces of the map that lead to it.
 * Manipulative Bastard - The villain, "Leroy", considering his true identity and the Chekhov's Gun he had planted from the start.
 * Multicolored Hair - Shannon has this frequently, as she often puts streaks in her hair, or even dyes it multiple colors. Heck, when she's kidnapped, the book mentions how her "red and orange hair" is messed up.
 * No One Gets Left Behind - They took a pyrate oath, after all!
 * Only Smart People May Pass - Or not get killed, as some clues lead straight to booby traps.
 * Parental Abandonment - Paul's backstory. His mom took him underground into the tunnels to escape his abusive dad, but later died when she didn't have enough food, as she was giving it all to Paul.
 * The Password Is Always Swordfish - Let's just say that once certain information on "Leroy" is discovered, the password to his computer becomes a lot easier to guess.
 * Photographic Memory - What Derrick has, which is put to good use at times.
 * Pirate - Whether Captain Kidd was type 1 (the evil type) or type 2 (the romantic type) is something George tries to find out, and receives conflicting information about throughout the course of the series. On the one hand, Kidd wrote a pyrate oath which includes a demand not to steal from "those who do not deserve it." On the other, he set booby traps designed to kill.
 * Police Are Useless - "Leroy" has insiders in the police force helping him, so the kids can't come to the cops for help when things get serious.
 * Reasonable Authority Figure - George's dad, once the kids have to tell him what's up in the fourth book, takes it surprisingly well. There's another pair of reasonable authority figures later on.
 * Ruritania - Litaria, a fictional country that was said to be a former part of the Soviet Union. Captain Kidd's hidden treasure is from that country, as is George's dad's girlfriend in the third and fourth books.
 * Schedule Fanatic - George's dad. This also works to George's advantage, when he can be guaranteed to be out of the house at a certain time.
 * Secret Underground Passage - One hidden under George's house, and "Leroy" has one leading to his.
 * Shown Their Work - The author did research on the history of actual pirates, as well as the building of New York City, and some of it shows up in the story, even if some other details are made up for the sake of keeping things interesting. But it's done in a mostly subtle way, except for the one point at which George explains to his friends (and by proxy, the reader) the origins of the skull and crossbones flag.
 * Sixth Ranger - Paul, who's technically not a member of the team, but helps (sometimes if persuaded).
 * Snooping Little Kid - Also results in the kids getting in danger, as you'd expect. They're not just snooping around in bad guy lairs, but also in the place where Captain Kidd may have hid his treasure, and definitely his traps!
 * Tomboy - Shannon. She's got colored streaks in her hair and leads her own sixth-grade band.
 * Trail of Bread Crumbs - Trail of cookie crumbs, left by Shannon as a hint for her friends when she's in the process of being led away by her kidnappers.
 * Treasure Map - Naturally.
 * Tunnel Network - Where much of the action takes place - in the abandoned train tunnels and caves underneath New York City.
 * Ungrateful Bastard - When Paul is first rescued, his initial response is to smack Renee's hand and punch Derrick hard in the gut, then run off. He later saves George's life and does thank him for the rescue.
 * Voice Changeling - George uses this skill several times.
 * Why Did It Have To Be Lakes - Paul has to confront his biggest fear in the third book.
 * Would Hit a Girl - "Leroy" doesn't discriminate in who he attacks, we learn in the finale.
 * Would Hurt a Child - "Leroy" would, and does, in the finale! With a knife!
 * You Meddling Kids - Invoked before being caught. "I'm not about to let my plans be stopped by a bunch of 11-year-olds!"