"The Practice of Celtic wisdom: Druid Magic" by Maya Magee Sutton and Nicholas R. Mann
"The Path of Druidry: Walking the Ancient Green Way" by Penny Billington - Great book by a Druid celebrant from The Order of Bards, Ovates, and Druids.
"The Mabinogion" - This is a must have for any druid or student of druidry in my opinion.
You can also check out the Order of Bards, Ovats, and Druids website and order a nice little intro package. It's affordable and I don't think the shipping to the US is very expensive.