Seriously, I don't know how he does it, but each one of his novels is humorous, fun, and profound. Paper Towns was no exception (obviously).
Quentin Jacobson has been in love with his next-door neighbor Margo Roth Speigelman since childhood, but by high school they are barely even friends, and definitely not in the same social circle. But one night, about a month before their high school graduation, she takes him on a midnight adventure and then disappears the next day. Quentin believes she has left him a clue to her whereabouts, and he and his friends begin to unravel the mystery of her disappearance.
Like Looking for Alaska and An Abundance of Katherines, Paper Towns is populated with a host of quirky characters (such as Q's friend Radar who spends his time obsessively updating the omnictionary) whose interactions with each other are complex and often quite amusing to the reader. The novel, while dark, contains just the right amount of comic relief -- along with the exact right amount of self-exploration and literary analysis (a Walt Whitman poem becomes an important part of the plot).
Very, very highly recommended, especially to those who enjoyed Green's first two novels.
Readers might also enjoy:
Thirteen Reasons Why by Jay Asher
It's Kind of a Funny Story by Ned Vizzini
My Most Excellent Year by Steve Kluger (just finished yesterday, review coming soon!)
Other bloggers' reviews (can you tell this one's popular?):
Avenging Sybil, And another book read..., Emily Reads, Just Listen, The Book Muncher, The Miss Rumphius Effect, Reading Keeps You Sane, a. fortis, Bookends, Bookshelves of Doom, Em's Bookshelf, Confessions of a Bibliovore, Guys Lit Wire, Liv's Book Reviews, Reader Rabbit, Plenty of Paper, Jen Robinson, Once Upon a Bookshelf, Frenetic Reader, The Compulsive Reader, YA books and more, Kids Lit, Reviewer X, Reading Rants
And for more evidence of John Green's brilliance, read his recent speech from the ALAN conference which he kindly posted on his blog.