You could try this: MP3Cutter 4 - The One And Only FreeWare MP3 Cutter

Note: It runs into permission issues on some computers and won't start, but it's standalone and freeware, so just try it and see if it works. If it does then it's the best tool for that exact action, since it does not decode and reencode. It just cuts and rewrites header info.

This is of course if the audio books are in mp3 format, as you didn't state what format they were in.