X-link to the DRM-free megathread where a couple of Humble Bundles offer a few nice non-fiction things amidst their general content:
Humble Total War Bundle offers at the minimum basic tier level two Osprey Publishing military history battle guides set during the time of the Roman Empire (Gaul and Germany), which are quite good quality as you might expect from their imprint. These are general quasi-scholarly books with proper bibliographical references, and not special gameplaying tie-in things.
Humble Book Bundle Neil Gaiman Rarities offers his extremely rare 80s-era Duran Duran biography for fans of the band, as well as some non-fiction essay pieces and speeches by him, mostly on the subjects of comics, fantasy writing, and censorship, amidst all the GN stuff.