Looks like they definitely just scanned some books. I downloaded a sample of "Information Theory and Statistical Learning" and the font was too pixelated and thus unreadable on my android device unless I zoomed in. Zooming in only evidences the pixelation even more.

An O'Reilly book seemed fine, so I want to imagine that technical/math-notation intensive books are hard to optimize for e-reading. Anyone have some knowledge about this? How hard is it to make a technical book's digital font easier to read if you don't already have the LaTeX/pdf?