If youre on a Mac, there are two decent scripts at Doug's AppleScripts for iTunes, but Audiobook Builder is a much better way to go, especially if you have a lot of books. Even at 1985 minimum wage, it'll save you enough time to pay for itself after 3-5 books.
If you're on Windows, there's a tool there, too, but I can't speak to it personally.
More details on the options available, and a whole lot more about listening to audiobooks on an iPod, can be found at Aldo on Audiobooks. In particular, there's a long-ish article reviewing the different tools for creating Audible-/iTunes Store-style "long" audiobooks (as opposed to one track per CD) at: How to join multiple tracks into a single audiobook file.