Take the Chestnut Hill West line (formerly R8) to Carpenter Lane station. Head downhill on Carpenter Lane toward Lincoln Drive. Cross Lincoln Drive, and the bookstore is two blocks further on Carpenter, near Greene Street. Look for the Earth flag. If you're at Weavers Way Co-op you've gone too far.
The H bus stops at Carpenter and McCallum, one block away from the store. The 53 and 23 buses also stop at Carpenter (on Wayne and Germantown Aves. respectively); they are a longer walk. Note that the H and the 53 connect to the Broad Street Subway line at Broad and Erie.