These are the three books Telltale is licensed to do games about. You can probably expect further books to come to the store when they grab the accompanying game licenses. Of course that would be after they've made Eyes of the Storm which isn't anytime soon.
I wouldn't worry too much about it going out of print. Bone seems to be a very popular comic series and that print run feeds a lot more stores than just Telltale's (for instance, you can find Amazon links through boneville.com