It should be pointed out that WavePad does have some file splitting features, just not one to automate splitting a file into specific lengths. Check out the 'Split' dropdown item on the 'Edit' tab. It has the following:
- Split file at cursor
- Split file at silences
- Split file at bookmarks
- Split file into its component channels.
The first three of these could be used to split a long audiobook into separate pieces. You could manually find points at which you want to split the file, and use 'Split file at cursor'. Or you could bookmark each place you want to split the file (use descriptive bookmark names, such as 'Chapter 2'). Or you may even be able to use 'Split file at silences', setting a low threshold and long duration. The low threshold says "don't split unless it's really quiet", and the long duration says "don't split except when there's a long pause". If your reader pauses at the end of each chapter then this may work.