At the ATOCHA subway stop (Salida: Ministerio de Agricultura) is the booklover's paradise known by the street's name "La Cuesta de Moyano". You walk uphill past LOTS of bookstalls, and at the end of the street is an entrance to "El Retiro". It's a great way to spend the day, or an afternoon.

I always make it a point to walk up/down Calle de Claudio Moyano 's "Feria del Libros" (click the 4-other views in the link to see the book stands). They have QUITE a selection of used (and some new) books. More than the books, I like to watch the people browsing. It's a true Spanish experience for me.

Saludos, MadridMan

Is it open every day?