We lived fairly close to the Cape when I was a little girl. We went often and stopped at Parnassus Book Service near every time. It is charming and the owner is kind and helpful. You walk in only to be surrounded by stacks and stacks of books shelved to the ceiling, in crates on the floor, and in the backroom if you are brave enough to peek. If you are there before or after hours, you can still shop. On the side of the building under the green awnings you will find paperbacks for a dollar and hardbacks as marked. Pick your treasures, slip the appropriate amount of money under the front door, and be on your way.
The Back Room