The bookstore to end all bookstores, at least in South America, is the majestic and stunning El Ateneo Bookstore in Buenos Aires.

http://argentinastravel.com/268/el-ateneo-in-buenos-aires-a-bookstore-to-end-all-bookstores/
The bookstore to end all bookstores, at least in South America, is the majestic and stunning El Ateneo on Avenida Santa Fe in Buenos Aires. Where else can you sit in a theater box and leisurely read a volume of Neruda, or sip a cortado where Carlos Gardel once performed? In a city with a rich literary history and excellent bookstores, this theater cum bookstore is a historical and beautiful building to visit, and a great place to stock up on books and music.