While The Complete Illustrated Encyclopedia of Magical Plants by Susan Gregg is probably my number one resource for plant and herbal information in magic, I would have to say that this is my number one resource for trees in magic. Alice Thoms Vitale is a wonderful author, and this book she has created is a work of art.
The book is a collection of trees and other leafy plants and all their wonderful information. It provides beautiful pressed scans (or maybe they’re very detailed drawings?) of each plant’s leaves, a detailed description of the plant and it’s role in the history of man, the scientific and botanical properties of the plant, the medicinal properties of the plant, and the magical and metaphysical properties of the plant.
While Susan Gregg’s book is an incredible source on herbs, this is an incredible source on trees. I think they are both incredibly useful and vital to my personal practice, not only as a witch, but also as an herbalist. I highly recommend picking up a copy if you are interested in incorporating trees into your practice, or are just interested in botany or green magic in general.