and at liberty (as I am these days) then definitely swing by the Brattle Book Shop on West Street. Their daily outdoor booksale (in the adjoining empty lot) is a thing of wonder with top prices on hardcovers reaching an affordable $5. Most tradepaperbacks can be had for $1 to $3 dollars maxiumum. If you were a penurious local undergraduate, then this might be a way to secure some assigned readings at prices below the campus bookstore.
I've been buying disparate volumes of Will and Ariel Durant's History of Civilization here as I have gaps in the collection.
Trust me, you'll buy something in this lot, there is just too much temptation.