iPad with specifically formated PDFs for rules and modules here.
After having used a laptop for GMing for years, I got an iPad for work and tried to play with it. I first tried to go the epub way, but I found that, in the end, iPad formatted PDFs are much easy to read and use.
iPad is so easy to use behind a small GM screen for GM books, and easy also to pass around players for maps or illustrations. Definition and brightness are really amazing when you zoom in the illustrations, with PDFs generated at a mere 150 dpi.
I had to reformat all my documents at a 1004x768 ratio. You also have to take into account some specific things, like no recto verso, small margins (but margins anyway) illustrations or maps at a fixed ratio (so that they fit screen perfectly when you zoom in), good index generation, etc. InDesign "screen" layouts does all this perfectly.
And, by the way, the generated PDFs are also very usable on a laptop screen, with the "1 page at a time" "fit page to screen" view. I don't use specific apps for now, as the system I'm using now (FATE, Legends of Anglerre style) doesn't require me to use one.