The Mechanics' Institute Library at 57 Post Street is a wonderful place. Don't let the name mislead you, it's a general interest private library with membership open to all. It's located in a beautiful building. The lobby was recently restored, but that isn't the nicest thing about this location; it's got to be the staircase. Elaborate spiral staircases may be a dime (or 7.4 euro cents) a dozen in Europe, but they're pretty rare in San Francisco. This would be a beautiful example anywhere.