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I've read two books:

Test Driven Development: By Example by Kent Beck and

Unit Test Frameworks by Paul Hamil

The Beck book is well regarded, but I didn't get started with unit testing until I read "Unit Test Frameworks". I do some TDD, but I also add tests to older code that I have to maintain (when I can).

Edit: Also, once you get a handle on it, I recommend using it on a current project right away. For me that's when the real learning occurred, and I think the "Unit Test Framework" book was a better reference book for this purpose. (I was using nunit with C#).