I have to admit I have an obsession with second-hand bookshops. I like the idea that other people may have read the same book and have experienced the same feelings as I have while reading it. One of my favorite ones is Re-Read. They have different spots in the city so you can always find one near you.
The place is very welcoming, with its warm lights and stacks upon stacks of books. They are divided in categories and you can find books in many languages besides Spanish and Catalan. If you are looking for a specifik book, you can use their online search engine which shows in which store you can find your book. What I usually like to do is wander around there for quite some time and pick up the books that look interesting to me.
The prices are pretty cheap and the more books you buy, the cheaper they get. You can get up to 5 books for 10€.