Missing audiobook purchases
Audiobooks are not included in my account under 'Purchased' as is music that I have bought. Why not?
Audiobooks (and tones) are a one-time only download from the store (and other items are only redownloadable whilst they remain in your country's store and you don't move countries) - I believe all audiobooks are supplied by audible.com, so I assume that it's them requiring the one-time only download,
Have you not a copy of them on your backup of your downloads/library ? If not (and if they are still in the store and you haven't moved countries) then you can try contacting iTunes Support and see if they will grant you a redownload : http://www.apple.com/support/itunes/contact/