to the OP:
I’m in the Windows realm, and sometimes use the Windows version of iTunes, so I can’t give you absolute specifics about Mac. But maybe this can get you started:
Tags are electronic labels in the files. They display things like Album, Artist, etc. The player reads them and makes lists from them. If it can’t read them, or they’re not there, you get Unknown.
Most of the Audiobooks you’re getting probably have the tags put in them correctly. But someone has to put them in there in the first place. If they didn’t, for the books that are giving you trouble, then you have to do it.
With iTunes you can highlight a file and under File, Get Info–which shows you the tags. Try it on one of your problem books and see what the tag info is. The Genre tag should be Audiobook–that’s how the Clip puts them in Audiobooks. See if anything else is obviously wrong, and change it.
The reason your books are alternating is that the Sansa is reading the Track Number tags. And both volumes probably have the same name (Book instead of Book Part 1 and Book Part 2) so the Sansa is playing both track 1s, then both track 2s, etc. The easy fix for that is to give the separate volumes different titles, like Book 1 and Book 2.
Using Get Info in iTunes, you can change the same tag in multiple files by highlighting a bunch of them and typing the correct info into the box. But it’s pretty clumsy.
Even if you don’t change anything, Save the tag. That could change the version of the tag and make it readable to the Sansa.
What that means is that through the years, the way the tags appear to the computer has been changing.
Sansa players read ID3v2.2 and ID3v2.3 versions of the tags. Newer tags may be in ID3v2.4, which will show up as unknown–and very old tags will be ID3v1, also unknown. iTunes doesn’t give you any flexibility on tag versions, but older iTunes gives you ID3v2.2, which should work OK. I don’t know about iTunes 11, one of the most horrible pieces of garbage software in the known universe. For all I know it gives you ID3v2.4, useless for our little Sansas.
In my Windows world, there is a great piece of free tagging software called mp3tag that lets you choose the version, change tags in bulk and automatically number them. But it’s not a Mac program.
I don’t know the Mac world at all but there should be taggers around. Here’s one that turned up on Google, and it looks like they’ve been working on it for a decade, so it ought to be good. It does change ID3 versions if needed.
No, you shouldn’t have to do this. But Apple loves to make its own devices work easily and make you jump through hoops for anything else.
If you do find a good piece of tagging software, you should be able to give it the right settings (tag version ID3v2.3 ISO-8859-1, which is the Windows encoding the Sansa prefers) and just run each book you download through it–maybe 5-10 seconds per book. It’s an inconvenience, but a pretty minor one.