Hey there. I found that learning about the disorder, how it affects people and how it's treated, was immensely helpful in understanding what my boyfriend's going through, so it's great that you're taking this step. I liked Break the Bipolar Cycle, because it's a good overview, an easy read, and I like the authors' perspective.
I also highly recommend talking to a counselor about what you're going through. Counseling can help you uncover what's going on in your own mind and clarify your own needs while being a strong partner. Good luck.