XANTH!! (27 books and counting) -Piers Anthony
Incarnations of Immortality (7 books) -Piers Anthony
Harry Potter (5 books, going up to 7) -J.K. Rowling
All of Amy Tan's books (4) (but I haven't read her kids' books, so I guess I shouldn't say all... mrgreen )
The Thief -Megan Whalen Turner
The Queen of Attolia -Megan Whalen Turner
The Enchanted Forest Chronicles (4) -Patricia Wrede
The Death Gate Cycle (7) -Margaret Weis and Tracy Hickman
The Princess Bride -William Goldman
Artemis Fowl (3 books, another expected) -Eoin Colfer
The Wish List -Eoin Colfer
Blood and Chocolate -Annette Curtis Klause (I'm working my way through The Silver Kiss, and Alien Secrets is next in line.)
I would put Eragon down, by Christopher Paolini, but I'm not finished yet. I would put J.R.R. Tolkien's books down if I weren't still slogging my way through The Return of the King.