UPDATE: The DVD is out, you can buy it on the official site
I randomly came across something called The Books - Playall. Downloaded it, started listening, and there are songs I heard ages ago at the "Forest of Sound" festival, which aren't on the old albums. Apparently this Playall is a DVD that supposedly hasn't even come out yet, but for some reason I downloaded MP3s.
Basically, nothing but mysteries, just as it should be for independent experimental musicians.