Numerous books have been published about Agile Development, unit testing, and specific unit test frameworks. A few of them are listed here:
Test Driven Development: A Practical Guide , by David Astels (Prentice Hall)
Test Driven Development: By Example , by Kent Beck (Addison-Wesley)
Test-Driven Development in Microsoft .Net , by James A. Newkirk and Alexei Vorontsov (Microsoft Press)
Testing Extreme Programming , by Lisa Crispin (Addison-Wesley)
Pragmatic Unit Testing in C# with NUnit , by Andy Hunt and Dave Thomas (The Pragmatic Programmers)
Pragmatic Unit Testing in Java with JUnit , by Andy Hunt and Dave Thomas (The Pragmatic Programmers)
Unit Testing in Java How Tests Drive the Code , by Johannes Link (Morgan Kaufmann)
Agile Software Development: Principles, Patterns, and Practices , by Robert Martin (Prentice Hall)
JUnit in Action , by Vincent Massol and Ted Husted (Manning Publications)