It’s too bad that Pagan Portal doesn’t have a book on Gaia. They really should! 
Assuming that you are looking for Gaia the Greek goddess and not Gaianism the earth-centered modern spiritual approach, here are a couple of interesting places to start by Theoi.com:
Here on Spells8: Gaia Symbols, Correspondences, Myth & Offerings 
I haven’t found any books specifically about Gaia like the PP collection but there are some good Greek mythology books that you should still check out:
- The Penguin Book of Classical Myths
- The Greek Myths by Robert Graves
- Mythology by Edith Hamilton