There are lots of excellent ones, actually, several of which we've reviewed for the
Bunkhouse. In no particular order, Gamma and Beck's "Contributing to Eclipse", Shavor et. al's., "Java Developer's Guide to Eclipse", Clayberg and Rubel's "Eclipse: Building Commercial-Quality Plugins" and Gallardo, Burdette and McGovern's "Eclipse in Action" are all really wonderful books. If you were to buy only one, I think I'd recommend Eric Clayberg's book; it has in-depth coverage and I liked the organization.