If it stays frozen, you may have to format it, which will erase anything you’ve put on it. But don’t format from Mac.
Find someone with a Windows computer, connect the USB to the computer, hold down the << and connect the Sansa. That should force a Windows connection, and in Computer (or My Computer) you can right-click on the Sansa and Format it to FAT32.
Library books usually go through consoles so they can transfer data like expiration dates and unlock codes. Drag-and-drop probably gave you unplayable, locked files. Audiobooks don’t take up that much room, so I would guess you have sent the Sansa something it can’t digest.
It looks like even though there is Overdrive for Mac, it’s still Windows-oriented.
That seems to mean that it’s going to depend on what kind of audiobook files your library offers. If it’s sticking to rights-protected Windows Media Audio files, you won’t be able to play them without getting a Windows computer involved.
On the other hand, if they are plain old mp3 files, you may not need the console at all, because there are no hidden codes to send. You should look at the tags in iTunes (Get Info) and make sure Genre says Audiobooks. If you’re not sure what format they are, you can highlght them in iTunes and Create MP3 Version. Then, just as an experiment, copy-and-paste some mp3s into the Sansa’s Audiobooks folder. It should work if they are mp3s without digital-rights garbage.