I've added a new (unofficial) category - Songbook(s). I've been adding it to songbook threads when I see them, and I've changed the term "song book" to "songbook" when I see that. Hope people don't mind, but it should make it easier to find songbook threads with the filter.
I fully realize that there is disagreement about whether one should say "folksong" or "folk song"; and "songbook" or "song book." Since "songbook" takes one fewer character, I arbitrarily decided on "songbook" - with no intention to slight those who believe the term should be "song book." Oh, and the category can be singular or plural, since the Filter picks up both if you enter songbook.
-Joe-