Unless you scroll down Music and go to Folders, the Sansa reads ID3 tags, not filenames. ID3 tags are the electronic labels in the file that name Album, Artist, Track and, most importantly for you, Genre (Audiobooks).
In iTunes you can look at Get Info. That tells you what is in the tags
You could fix them one by one with Get Info. That would be tedious.
So take them off the Sansa and fix them on the computer.
Since you are in Mac land you should probably get kid3, a free tag editor.
Install it, open it, go to Settings and Configure Kid3. You only need to do this once. Under Tags you’ll see Tag 1 and Tag 2 tabs. Open each one and choose ISO-8859-1 for text encoding. That’s Windows text display, different from Apple. Also choose ID3 version 2.3. Make sure you save those.
Use File to open an Audiobooks folder. Highlight them all, make the Genre Audiobooks. Make any other changes that seem sensible to you: make sure the track numbers are in order, etc. Save them, which will make them ID3v2.3 ISO-8859-1, Sansa-friendly.
This sounds complicated, but once you have Kid3 installed, all you need to do is open an audiobook folder, do a bulk fix of Genre or whatever else you need, Save it and send it over. Less than a minute for each new book–and probably more like 20 seconds.
Don’t forget to put the cap on the valve after you fill the tire 
