Through projects like https://www.gutenberg.org a lot of old books that are past their copyright period (often 70 years or more old) are being made available free online. For many students this is an opportunity to get books they could not otherwise afford. Do you know of free online books that are relevant for students? One example from my own perpective is the wonderful book about experiments with developing excellent learning inviroments https://www.gutenberg.org/ebooks/48906