If you use an eReader, I'm sure you know the difference between a PDF and ePub/mobi book. In the case of the Kindle, for the latter, you can change the font and its size, look up words in the dictionary and even have the Kindle read the book out loud to you. For a PDF, you are normally forced to resort to landscape to get a reasonable text size and to not have to use the cumbersome zoom feature.
There are plenty free eBooks about programming in PDF and/or HTML. The latter can easily be ported to the eReader formats and some people have indeed been doing that. Here I've tried to list the free ePub mobi programming books that I've found on the web (mainly in GitHub). Note that most eReaders do not support both formats, but that usually isn't a problem, because you can use software like Calibre to convert between the two. Credit goes to authors and porters. Happy reading!
And if you know of some more, please add them in the comments!