Shopgirl by Steve Martin (130 pages) -- A bittersweet coming-of-age comedy about three people terrified of adulthood. If you've seen the movie (starring Martin, Claire Danes and Jason Schwartzman), it wasn't quite as good but faithfully captured the tone of this book.
The Westing Game by Ellen Raskin (182 pages) -- A hilarious parody of whodunit stories that subtly morphs into an interrogation of the American Dream, as sixteen amateur sleuths seek to make sense of the world's strangest will. Written for children but no less rewarding for that. Come for the comedy, stay for the heartbreak. This is arguably my favorite novel of all time.*
*Arguably, because Madame Hoo is a character that exists and for a book that sets about creating stereotypes and then subverting them, this one fails hard with this one character. A minor gripe in an otherwise brilliant piece.