I just had an email in my inbox recommending "The Art of Unit testing" by Roy Osherove. I guess I’ll put in another order at Amazon. 
I've been doing unit testing for a long time, but since I'm largely self-taught I can probably learn something. Perhaps I’ll even sign up for a course at DevelopMentor. I know all the basics, I feel the biggest challenge with unit testing larger applications is managing test data.