Cuesta de Moyano
Time Out says
Claudio Moyano street, also known as Cuesta de Moyano because of its slope, is one of the most famous streets in Madrid not only because of its location between Atocha and Retiro, but also for the book booths along the fence of the Botanical Garden. It's perfect if you're looking for an old, second-hand book, and sometimes you can find truly authentic gems. Skaters often frequent this wide pedestrian street with benches.