Non-Violent Communication - Marshall Rosenberg, Ph.D.
The Getting Real series - Susan Campbell, Ph.D.
The Five Love Languages - Gary Chapman, Ph.D.
Because so much of our communications challenges are centered around fear, I'll include my tried and true fear reference as well, which serves well for any personal challenges where fear is a major factor.
Feel the Fear and Do It Anyway - Susan Jeffers, Ph.D.
Note that many of these authors offer workbooks for and audio recordings of their books and/or workshops that function as highly useful companion tools for implementing lessons learned. Also, the skills learned are valuable not just in making polyamorous relationships work but for all the important relationships in our lives.
What other communication skills resources do you recommend? A comment here or an e-mail to firstname.lastname@example.org would be most welcome.
Hope this helps!