Books on specific subjects are often varied, numerous, and for the reader are usually a matter of taste. Although there are a great many books available on GM, included below is recommended reading of books on GM which this specific reviewer has found very useful:
Green Man: The Archetype of our Oneness with the Earth’, by William Anderson and Clive Hicks Harper Collins 1990
The Green Man. Kathleen Basford. D. S. Brewer, 1978. Highly recommended, probably the first serious study of the Green Man including the classic three types, and includes many fine illustrations.
A Little Book of the Green Man. Mike Harding. Aurum Press, 1998
The Green Man: A Field Guide. Clive Hicks. Compass Books (2000).
Highly recommended. The book to start with, a gazetteer; list where to find GM by county, country, and includes Europe, Public House with the name The Green Man, exhibitionists, Sheelas etc.
Explore Green Men. Mercia MacDermott Heart of Albion Press 2003
Highly recommended, a book that come closest to the theories and source of GM.
The Quest for the Green Man. John Matthews. A Godsfield Book, 2000.
But please do not take this reviewers opinion, read around and select books you feel most comfortable with.