Books, PDFs Playlists?
Working with a script to get all the names of the playlists in iTunes, I noticed that two playlist names popped up in the latest iTunes 12.1 that aren't actually visible in my Playlists: "Books" and "PDFs". Here's a script that illustrates:
tell application "iTunes"
get every playlist whose name = "Books"
--> {user playlist id 60870 of source id 80}
get every playlist whose name = "PDFs"
--> {user playlist id 60873 of source id 80}
end tell
While I almost certainly have had "PDFs" playlists in the past, this doesn't appear to be some artifact from a past library. These playlists aren't in the XML either, but they have persistent IDs and low index numbers. I tried using AppleScript's delete command to remove them but it didn't work, which leads me to believe these are hard-coded and are supposed to exist. Not sure what this is about (iTunes does use invisible playlists from time to time) but it may be inconvenient and inaccurate if you use a script that lists playlists.
UPDATE: Kirk speculates that ebooks may be returning to iTunes.