There have been comparatively few books written about the Green Man, and few of these have anything worth saying.
They tend to go out of print quickly and can be difficult to obtain but here are some currently available from Amazon.
The best, by far, is Kathleen Basford's The Green Man, it contains many fascinating illustrations and avoids unfounded speculation. Unfortunately, the latest edition has been produced on a digital press and the pictures while excellent are not as clear as before