TDD by Jason Gorman is a nice (and free) introduction: http://codemanship.co.uk/tdd_jasongorman_codemanship.pdf (disclosure: I did Jason's ~TDD training course a number of years ago and gained a lot from it - but the book is useful on its own).

A book that explains TDD in the context of a much larger example application is Growing Object-Oriented Software, Guided by Tests by Steve Freeman, Nat Pryce (sometimes referred to as "The GOOS Book").

One of the difficult things about TDD is that it takes a long time to learn to do it effectively. I personally didn't really "get" TDD until I paired with a few much more experienced developers, and this forced me to rethink the way that I approached writing software. Ultimately, it caused me to significantly level up my coding skills and I wouldn't go back to the way that I used to write code before.