Anything by Patricia McKillip - she really emphasizes the power of words, meaning, and intention. Plus, her prose is like poetry. She's a bit of an acquired taste - according to the book club where I tried to introduce her - but I think she might appeal more broadly to druids. I hope so!
I also really like Susan Cooper's The Dark is Rising
Sequence for the way she uses mythology and her sense of atmosphere. I've read them several time, both as a child and as an adult, and they still ring true to me.
Pamela Dean wrote the books Tam Lin
and Juniper, Gentian, and Rosemary
- and there's a certain, slow-building sense of enchantment in those books that I highly recommend. Fire and Hemlock
by Diana Wynne Jones is one I recommend, as well, because of the way she tells the story ... it's another version of Tam Lin.
I think most of the stories I really like are about transformation and story-telling. They're really very bardic in nature! I know several of my choices are generally considered Young Adult, but I think they overcome that category by being really good tales. Still, you could let a young person read almost all of those (except maybe Tam Lin by Pamela Dean ... it's a bit older in theme).