People have already listed them, but:

- Crucial Conversations

- Difficult Conversations

- Nonviolent Communications (aka NVC)

The first two books are great at explaining the dynamics at play, but poor on actionable advice. The third book is great at actionable advice, but poor on explaining why the advice is good. So I'd recommend reading at least one of the first two books as well as the NVC one.

There is also, essentially, an NVC cult out there (reminiscent of agile cults or TDD cults) - in the form of NVC chapters in each city.. Ignore them - just focus on the book. In my experience, local NVC chapters always seem to add more to the material in the book, and often follow a world view not particularly espoused by the book - one which often repels people.