The Book of Celtic Symbols: Symbols, Stories, and Blessings for Everyday Living by Joules Taylor is a comprehensive primer for those new to the Celts as well as a concise work that provides insight into Celtic life for those of familiar with these people.
Deva: Our Relationship with the Subtle World by Jacquelyn E. Lane is a title that anyone who is serious about the undertaking of engaging and communicating with the natural world should read.
Finding Home within the Heart of the Earth by Eagle Skyfire felt more to me words to be practiced rather than a book to be merely read.
Goddess Love Oracle by Wendy Andrew is an incredible deck that packs a lot of Goddess guidance into it.