Growing Object-Oriented Software, Guided by Tests, by Steve Freeman and Nat Pryce, ISBN 0-321-50362-7
Freeman and Pryce explain Test-Driven Development through an extended example. (They have a somewhat different perspective than I do, with much heavier use of mock objects. I’m not sure what drives this difference; it may be due to type of application, philosophical reasons, or just something I should learn.) I love the clarity with which they tackle the problem of driving in from all the way outside, and how they don’t shy away from dealing with concurrency and persistence. It’s high praise for me to say a book deserves further study; this book is one I’ll definitely be reading again.