YA Books for Adults

The Age of Miracles by Karen Thompson Walker
Despite its teenaged protagonist, this book doesn't feel like a YA read. It explores what would happen if the Earth began to rotate more and more slowly and is haunting and memorable.
Boy by Roald Dahl
Dahl's memories of his childhood are fascinating and all related in his inimitable style (with hysterical pictures by Quentin Blake).
Wonder by R.J. Palacio
Achingly beautiful story of a middle school boy with a rare and profoundly severe facial deformity. It will probably make you cry a few times, but by the end they will be happy tears. 
The Book Thief by Markus Zusak
Stunningly unpredictable (the narrator is a sort-of-personified Death) and beautifully written, about a German girl living during WWII with sticky fingers where books are concerned. 
Code Name Verity by Elizabeth Wein
Starring one of my all-time favorite characters, this fresh book is full of twists and turns that I can all but guarantee will leave you guessing until the last page. And who doesn't love a good female spy?
The Absolutely True Diary of a Part-Time Indian by Sherman Alexie
An adolescent Native American straddles two cultures: the reservation, and the off-rez all-white high school where his intellectual prowess has earned him a spot. Funny and moving. 
Haroun and the Sea of Stories by Salman Rushdie
Delightfully imaginative, fairy-tale-like adventure story that's also complex and sophisticated. (This is Rushdie, after all.)
The Hobbit - by J.R.R. Tolkein
High adventure abounds in Middle Earth when a lovable hobbit teams up with a band of dwarfs to win back their kingdom from a dragon. You'll be sucked into the rich, lush world Tolkein has created.
The Hitchhiker's Guide to the Galaxy by Douglas Adams
Outrageous and hilarious, this story of intergalactic snafus is a little bit sci-fi, a little bit humor, and wholly enjoyable.
Catcher in the Rye by J.D. Salinger
Holden's voice, at once brash and vulnerable, has touched millions of young people. This timeless story feels as fresh as the day Salinger finished it.
The Outsiders by S.E. Hinton
Classic story about the bonds of friendship and loyalty. Ponyboy Curtis and his gang of friends clash frequently with the rich kids across town, but learn they're not really so different from each other. 
The Phantom Tollbooth by Norton Juster
This quirky, amusing tells how a trip to the world on the other side of the tollbooth pulls Milo from the depths of ennui. Delightful plays on words abound throughout this fun tale.
Hatchet by Gary Paulson
A white-knuckle survival story. A plane crash maroons young Brian alone in the Canadian wilderness with only his hatchet to help him obtain food and shelter. A pulse-raising, thrilling novel. 
Watership Down by Richard Adams
Hard to believe a book about talking rabbits could be this dark and nuanced. Read it to believe it. 
Seedfolks by Paul Fleischman
Seedfolks is composed of a series of short vignettes that tell the story of how an abandoned lot in a bad neighborhood becomes a community garden that improves everyone's lives. Warm and lovely.
The Scorpio Races by Maggie Steifvater
Puck, an appropriately impish teenage girl, and Sean, a talented and reticent jockey, both tell this tale of the dangerous annual race on their island, where mythical, vicious water horses are real. 
Between Shades of Grey by Ruta Sepetys
Inspired by the real, tragic events that befell Sepetys's own family in the 1940's. Lithuanian Lina's family, considered dangerous intelligentsia, must survive deportation to Siberian work camps. 
Pirates! by Celia Rees
What to do when your greedy father tries to marry you to a hideous cretin far too old for you? Run off with your slave-turned-best-friend and join a pirate crew! This book's quality belies its bad title. 
The True Confessions of Charlotte Doyle by Avi
Charlotte is a typical, well-bred girl, but things take a turn for the unexpected when her sea voyage in 1832 is interrupted by mutiny aboard the ship. A taut, riveting story.
The Wednesday Wars by Gary D. Schmidt
A middle school boy's nightmare come true: Wednesday afternoons alone with his hard-as-nails teacher while the rest of the class is at Hebrew school/confirmation classes. Charming and funny.
Feed by M.T. Anderson
Chilling, futuristic story in which all the characters are hooked up to "feeds" that allow them to seamlessly interact with the world without the need for computers. 
House of the Scorpion by Nancy Farmer
The protagonist of this story is a clone, bred to provide donor organs to his originator, but he throws a wrench into the works by escaping.
Harry Potter series by J.K. Rowling
Just as good as people make them out to be from beginning to end, and well-written to boot.
The Hunger Games by Suzanne Collins
Every adult I know loved the first of these books, liked the second, and was luke-warm about the last. I agree. Still, this post-apocalyptic series about violence, freedom, and reality T.V. is worth reading.
The Chronicles of Narnia by C.S. Lewis
This wonderfully imaginative series is filled with deeper symbolism, and Lewis's artful prose hovers miles above most "children's" authors' writing. 
The Calvin and Hobbes Collection by Bill Watterson
Wiser and funnier even than you remember. You'll return to this collection again and again.