What books would you recommend to someone interested in Druidry? I'm not looking for books specifically about Druidry, although you can mention those if you like. Rather, I would like to know about books that are on other topics, but that you think someone interested in Druidry should read. I would like it if you would also give the reasons why you are recommending each book.
Here are some examples:
The aspiring Druid should probably read some books on Celtic mythology. Although modern Druidry probably has little in common with the actual practices of the ancient Celts, much of the inspiration for Druidry today comes from Celtic myths and practices. Alwyn and Brinley Rees' Celtic Heritage seems to be fairly widely accepted. I also like Marie-Louise Sjoestedt's Celtic Gods and Heroes. As for the myths themselves, I like Patrick K. Ford's translation of The Mabinogi and Thomas Kinsella's translation of The Tain. Although Lady Gregory's retellings of the Irish myths in Cuchulain of Muirthemne and Gods and Fighting Men may not be the most scholarly versions, they are still a good read.
How about some field guides? Modern Druid is very much a nature spirituality, and one way to become more attuned to the natural environment is to begin identifying and learning about the living things that inhabit that environment. I can't really name any specific titles, because of course you will want to choose the ones that are specific to your region. Try to choose the most region-specific field guides as possible (such as for your specific province or state rather than for all of North America) as they will probably be more detailed and may make identification easier because they will contain less species.
Henry David Thoreau's Walden is a classic of nature writing, and is essential reading for anyone who loves nature. I also enjoy Edward Abbey's Desert Solitaire, Annie Dillard's Pilgrim at Tinker Creek, Aldo Leopold's A Sand County Almanac, and Peter Matthiessen's The Snow Leopard.
As the aspiring Druid begins her study of nature, she may wish to keep a journal of her discoveries, and Clare Walker Leslie's Keeping a Nature Journal may prove helpful.
Druids who want to follow a more Bardic path may want to read books on poetry and writing. I have found The Practice of Poetry by Robin Behn and Chase Twichell, The Poetry Home Repair Manual by Ted Kooser, and Steering the Craft by Ursula K. Le Guin useful. All of these books are practical than inspirational, because while I find that I have many ideas, it is more difficult for me to actually translate these ideas into poems or stories.
Finally, I would like to mention Always Coming Home, which is a unique work by Ursula K. Le Guin that provides a glimpse into the life of a people who live close to the land and whose spirituality is inseparable from the rest of their lives. I don't think that I have ever read this book without wishing that I could live with them.
What books would you recommend to the aspiring Druid?