Q: I purchased a audiobook on iBooks a couple of weeks ago, and know it says I haven't purchased the book?

Mine don't show as purchased in the store either (so you will need to keep tracks of which that you've bought so that you don't accidentally repurchase one). Also audiobooks are a one-time only download, so even it did show as purchased you won't be able to redownload it without paying again

What have you done since you bought it e.g. updated the iOS version on the device via a computer's iTunes without copying your purchases over to your computer ? If you hadn't copied it to your computer's iTunes so that you could take a backup copy then you could try contacting iTunes Support and see if they will grant you a redownload : http://www.apple.com/support/itunes/contact/