Comment by bonoboTP

The machine learning, computer vision and robotics communities are really great at publishing their books online for free access. You can get the absolute top textbooks of these fields for free online. Quite a contrast to other fields where profs kinda require you to buy the latest edition for hundreds of dollars in the US. Not to mention that this gives access to the best resources everyone around the world in poorer countries as well. Many also share their course materials and videos online.