Do you discuss testing approaches in the book (e.g. TDD)?
Yes. Although I am a TDD practitioner myself, _JUnit Recipes_ is
not a TDD book. Some of the recipes are written in the TDD style, but one of my central goals was not to try to sell TDD. I make mention of TDD, refactoring, and the effect of tests on design without preaching about them. There is also plenty of information for people inheriting legacy code who need to test it.
What about testing topics such as what is enough testing or how to organize tests?
The question "what is enough testing" is handled throughout: there are some places where I discuss the ROE (return on effort) for certain kinds of tests, such as for constructors, get/set methods, and so on.
As for how to organize tests, see chapter 3.
Or are the recipes more geared toward technical solutions (and not philosophy)?
You can't get a book from me without a good heaping of philosophy. That said, I tried to present my opinions and my experience without lecturing. The readers can tell me the extent to which I succeeded there.