Books on Unit Testing and TDD?

I'm in a situation where I understand the importance of unit testing and how TDD works, but in practice I don't actually get to do it much. The technology I'm working in isn't really testable in this way.

That being said I'm interested in locating some reading material on these topics that don't just cover the basics, but do so with a little more depth so I can hopefully glean some of the senior level experience I'd get if I was actually doing it regularly.