North Beach's historic Beatnik bookstore stays fresh with a great selection of material and a comfortable (well lit!) atmosphere, including a poetry room on the top/3rd floor. For those digging a Beat-ly break from the books, Vesuvio awaits just across Jack Kerouac Alley :)
Bookworms in SF should also check out the larger and lower-key Green Apple in the Inner Richmond.
Address: 261 Columbus Ave, SF
Website: http://www.citylights.com/