What is a librería de viejo, and where are they found? Write your answer in English.

A librería de viejo is a second-hand bookstore or antiquarian bookstore that sells used and rare books. These types of bookstores can be found in various locations around the world, usually in larger cities or towns with a strong literary culture. Librerías de viejo are popular among book collectors and enthusiasts looking for unique and hard-to-find titles.