This is a very useful book; it will get programmers and testers up to speed with current thinking and practices of agile testing. One important topic that is discussed is the Testing Pyramid, invented by Mike Cohn. It is a model that helps you make sense of different kinds of tests you could write: from the tests that are closer to the user, to the tests that are closer to the code. The Pyramid reminds you that unit, integration and UI tests are useful in different ways, and have different strengths and weaknesses. All of this is discussed at length in the book.
Although this book is focused on testing for web applications, it should prove useful for people using other technologies, such as mobile development. The details are not the same, but the underlying principles about the nature of UI, integration and unit tests don't change.
This is a book about testing, not about Test-Driven Development; the latter is discussed in the final chapter. While testing is useful for both programmers and testers, TDD is mostly useful for programmers. For this reason, I suggest programmers to read more about TDD after this book. I suggest Kent Beck's "TDD By Example."
Full disclosure: I was a reviewer of the book before it was finalized.