According to that article you can not redownload audiobooks like you would everything else
http://support.apple.com/kb/HT5085.
So in the future I would suggest to make sure you backup everything to the computer.
But in the meantime you can email itunes to regrant that book to you here.
http://www.apple.com/support/itunes/contact/
Click "contact itunes store support" and email about your book.