Where are you seeing that says that they aren't available ?
Where did you buy the audiobooks, on your phone or on your computer's iTunes ? If you bought them on your iPhone then they should have downloaded into its Music app, the iBooks app is for reading books.
Note that audiobooks are currently a one-time only download from the store (so they won't show in the Purchased tab in the iTunes app on your phone for re-downloading), so you may want to copy it to your computer's iTunes via File > Devices > Transfer Purchases so that you can take a backup copy of it - it should copy over to the Books part of your computer's iTunes library (Audiobooks if using a Mac with OS X Mavericks+ on it).