Don't go wild buying textbooks/study guides, but getting a little extra reading material or some reference books isn't a terrible idea. You know, something to break up the monotony, or go to when you need a specific answer quickly (and either don't want to wade through the textbook lessons trying to find it, or it's a more advanced/obscure topic that isn't covered).
A bunch of reading material
Japanese Sentence Patterns For Effective Communication
Stuff like that is good to have on hand.