City Lights is one of the landmarks of San Francisco. It's an independent bookstore specialized in world literature, arts, politics and alternative culture. If you love books, this is one of the best bookstores in the world to visit.
Amazing bookstore not only for being a historical reference point for the Beat Movement (the owner went on trial for obscenity after publishing Ginsberg's Howl), but it manages to stay relevant, have a great selection of books and interesting events with friendly and intelligent staff. Check their site for events and readings: http://www.citylights.com/bookstore/?fa=books_events
Allen Ginsberg read his poetry there. If you are hungry, go across the street to Rose Pistola:
But bring a crowd - it is family style - huge portions.