There are two USB modes on the Fuze. Your computer can only see one at a time.
One mode is MSC, which is the universal USB port. The other is MTP, which allows the unit to be controlled by Windows Media Player. That’s Windows as in…not Mac.
You are caught in the compatibility wars between Apple and Microsoft.
If you look at Settings/System Settings/USB Mode you’ll see MSC, MTP and Auto Detect. Auto goes to WMP if it detects Windows Media Player 10 or above on your computer.
The idea was to have a user-friendly interface like iTunes to iPods for Windows users. Meanwhile MSC could connect to Apple and Linux (or allow direct manual control of the Fuze via Windows, which is what I do).
MTP is also necessary for digital-rights-protected files like library books because Windows Media Player/MTP also send over the hidden codes that unlock the files.
So if you were using a recent Windows computer and left your Fuze on Auto, your books were going over via MTP mode. Your iMac is connecting in MSC mode, so it can’t see them.
I suggest borrowing a Windows computer, taking the books off the unit (and anything else you may have sent over via MTP, like music) and putting them on a flash drive or portable hard drive. Then switch to MSC mode (Settings/System Settings/USB Mode) and reload from either Mac or Windows. That will make your files visible from any computer.
OOPS–one more hurdle. You mentioned the books are .wma. Windows Media Audio. Mac will not be able to play them, though it should store and transfer them OK.
If you want to play them on the Mac, you’re going to have to convert them to mp3.
Media Coder http://www.mediacoderhq.com/ is free, though you may have to use it on a Windows computer. When you install it, make sure it doesn’t install any toolbars or junkware–you probably have to uncheck some boxes. There’s probably a converter for Mac, too–Google around, or try Cnet.com downloads.
.wav audiobooks would be gigantic files, 10 times the size of .wma or .mp3 . There’s no point in having such a hi-fi file for spoken words. You should be able to find .mp3 audiobooks, though.