Richard Feynman - The Feynman Lectures on Physics (1964)
This blog is devoted to public domain titles that can be downloaded but this is a worthy exception. Caltech has made all three volumes of Feynman's classic available to read for free. The catch is that you have to read on the website. The experience is predictably a bit clunky though it appears to be done as well as it could have been - the images are clear and appropriately placed, the text seems to scale correctly, the navigation bar stays in a fixed location.