I'd recommend "xUnit Test Patterns: Refactoring Test Code" by Gerard Meszaros. It is not Python or Web specific, but it's a good book on TDD in general and the xUnit framework in particular. Since python unittest is actually an xUnit implementation ("a Python version of JUnit", as the docs say), I'd say that the book is very useful for Python unit testers.
It has an online version at xunitpatterns.com.
