Try L-Systems.

I've found "The Algorithmic Beauty of Plants" (linked to at the bottom of the page sunky provided) to be a pretty solid book, though it's written very much like a slightly dated thesis. It'll go in depth on a few topics, but a lot is left to outside resources.

L-Systems in and of themselves are cool (and can be used for much more than trees), but they're not the bulk of the work (and depending on your needs you can get away with faster, not-as-robust solutions). The bulk of the work will be building the final geometry, texturing, adding leaves, and lighting in ways that look good. There are lots of good papers floating around (the algorithmic botany site has some, and you may just want to search Google Scholar for more up-to-date papers if you can).

More OpenGL Programming also has something simple to start with, but the results look... less than amazing.

Cheers,
--Brian