Q: audiobooks disappeared

Audiobooks are a one-time only download from the store, so they won't show in the Purchased section for redownloading. Depending upon what your sync selection was when you synced yesterday you may have effectively told your computer's iTunes to delete it from your phone. Have you got a copy of the audiobook in the Books (Audiobooks if using an up-to-date Mac) section of your computer's iTunes library so that you can sync it back ? If not, and if you don't have a copy of it in your backup of your downloads/library so that you can add it to your computer's iTunes and sync it (and if it's still in the store), you could try contacting iTunes Support and see if they will grant you a redownload : http://www.apple.com/support/itunes/contact/