What if I purchased an audiobook and accidentally deleted part one?

Sorry, but audiobooks cannot be redownloaded. If you didn't make a backup, you may be out of luck and will have to repurchase the audiobook, though you can go here:

http://www.apple.com/support/itunes/contact/

and follow the instructions to report the issue to the iTunes Store. Perhaps they can make an exception for you.

Regards.