You should be able to get (illegal) PDFs of most popular books with a simple Google search. I found a PDF of the ML book I mentioned earlier as the top result on Google for "<name of book> pdf".

Admittedly epub is a better format, because it naturally reflows on smaller screens, but "free" epubs are harder to come across. I've been thinking of converting some really good PDFs that I have, to ePub myself, but just haven't gotten around to it yet.