I love reading! In addition to visiting Graz’s public libraries, I often stop at one of the many ‘little free library’ shelves dotted around Graz. You have probably already seen a few but here is a map showing them all (almost 80!). It’s a world wide movement encouraging people to share their books. You’ll find shelves out in the open, in cafes, or tucked into corners. Take a book, read and return it to a shelf somewhere.
Other places to find affordable second hand books are at Carla charity stores (they often have a foreign language shelf, especially English).
Books4Life also has a small store near the university and holds bigger book sales every few months.