Our frequent finds at the Book Bay Main, a snug used bookstore operated by the Friends of the San Francisco Public Library, tucked just inside the Grove Street entrance of the main library branch in Civic Center, led us to take a trek to its other location, the Book Bay Fort Mason. We stumbled upon a real treasure (containing lots of other real treasures). The store moved to new quarters in December, more than doubling its retail space (to more than 2,000 square feet). Fort Mason is a delightful place to visit, given its matchless seaside location, but you'll be tempted to spend hours inside, browsing the collection of well-arranged books. The selection ranges from rare editions housed in locked cases to brand-new review copies at less than half off their cover price, and tables of aging hardcovers and paperbacks priced at 50 cents each or three for a dollar (where we scored a copy of Alexanderplatz, Berlin, in print at $18.95). In addition to books, there's an astonishing array of ephemera: old theatre programs, posters, prints suitable for framing. And CDs, videotapes, and DVDs are also priced to move.