My university regularly put discarded books in a cart in front of the library with a money box, so you could just take a book and put one or two euros in the box. Among other things I got a copy of Benoit Mandelbrots "The fractal geometry of nature" which I still treasure to this day.

That's one of my treasures, too. I paid full price for a hardcover. Many of our books come from second hand dealers, often nature/history/tooling/photography foci. I'm a sucker for thematic histories, large format photography books, and well researched non fiction in general. If you like the marks on books, try postcards! I have ~20,000 of them. Not really sure why, but they're very interesting to scroll through. A card is like a little clue in to a time and place you would never otherwise think of ... logistics networks, handwriting, language, printing technology, inks, subjects of interest, urban development, architecture, fashion: sometimes with messages to boot!

Today's his birthday!