Harry Potter and the Prisoner of Azkaban by J. K. Rowling
Anyone who enjoyed the first two Harry Potter books should also like Prisoner of Azkaban. It has an excellent plot with lots of fascinating new creatures and characters. The pacing is very good--fast moving but never confusing. Like most of the Harry Potter books, nothing is really as it seems and the ending twists in a completely unexpected way. There's lots of humor too. The only thing I didn't like (which annoys me in all the Harry Potter books save the first) is that the descriptions of characters are repeated and events from the first two books summarized again when they come up. There are many continuing plot and character threads from the first two books, and the Harry Potter series is clearly intended to be read in order from beginning to end, so I really don't feel this repetition is necessary (it's easy to skip over, though). Speaking of the fact it's a series, don't buy this book if you haven't read the first two.