Tuuu Librería is a secondhand bookstore chain in Spain with a special twist. Operated by a non-profit foundation dedicated to literacy and reading, the bookshops collect and recycle donated books from individuals, businesses, and institutions. But what is really unusual is that customers only pay what they wish for the books they choose. In fact, the transactions are anonymous, with payments slipped into a box.

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There are currently Tuuu bookstores in Madrid, Salamanca, and Barcelona, each is staffed by volunteers. The organization also distributes books to schools, hospitals, senior centers, and jails. While the foundation has used the “profits” from bookstore sales to build libraries in South America.

So, if you are traveling in Spain, you may want to drop off your books after reading instead of just abandoning them in your hotel or Airbnb.