I think the first exposure I had to polyamory was on Wikipedia, but what really piqued my interest was a couple of articles from LessWrong, a rationality website.
"Rational Romantic Relationships, Part 1: Relationship Styles and Attraction Basics", by Luke Muelhauser
Covers a number of different relationship styles including polyamory, as well as attachment styles in general.
"Polyhacking", by Alicorn
How she went about deprogramming jealousy in order to be with the man she loved.
Already suggested are 'The Ethical Slut' and 'Sex at Dawn', both of which I've read and enjoyed, though I might have some reservations about Sex at Dawn that I could go into if I had the book in front of me (and if anybody cares :P)