Difficult Conversations, How to Discuss What Matters Most · Douglas Stone, Bruce Patton, Sheila Heen (Penguin 1999) · Great book, easy to read, lots of good examples from everyday life.
Getting to Yes, Negotiating Agreement Without Giving In · Roger Fisher and William Ury (Houghton Mifflin 1981) · Harvard Negotiation Project; excellent and short classic, primarily focused on business negotiation; explains interest based negotiation.
Getting Past No, Negotiating Your Way from Confrontation to Cooperation · William Ury (Bantam Books 1991) · Sequel to above, good advice on dealing with difficult people.
That’s Not What I Meant! How Conversational Style Makes or Breaks Your Relations with Others · Deborah Tannen, (Ballantine 1986) · One of several bestsellers on gender, cultural, and workplace communication by a linguist.