The Girls -- Amy Goldman Koss
Middle schoolers Maya, Renee, Darcy and Brianna all revolve around Candace. Candace is funny, smart, talented and, unfortunately for Maya, flighty and mean. When Candace decides that she doesn't like Maya, the rest of the girls follow suit. Maya finds herself floating on the outskirts of cafeteria society--when she isn't being tormented by Darcy, Candace's pitbull-ish sidekick. Written from the perspective of all five main characters, it's one of the better books I've read that deals with this issue.
Ugh. It brought back some painful memories. Since there are a lot of books that deal with a girl getting ditched by her friends in middle school, I'm assuming that I'm not alone.