Hardcover, softcover or digital?
For me, it has to be physical over digital. I really, really want to prefer digital because it would make life a lot easier and save so much shelf-space, but I'm just too used to print. My Sony e-Reader is pretty good, but the screen's too small to enjoy most roleplay books and it's an early model, so the battery doesn't last and (ridiculously) you can't read it while it's charging. But phone, tablet, or laptop are all too bright for me to read from, or the wrong shape, or something else I don't understand yet.
Beyond that, I don't have much preference. Hardcover books are great for using as a surface to make amendments to character sheets or on which to roll dice, but seeing as I have a specific hardcover folder for character sheets and a homemade rolling surface for my dice ('secret santa' gift from my group last Christmas) this doesn't really affect me, and as we have a table to play on at home, it tends not to affect my guests either. Hardcovers are more sturdy so less likely to get damaged when being carted between sessions - but it's very rarely me doing the carting, and anyway softcover books are lighter so easier to manage.
I do quite like the smaller, reference size books some games do, if for no other reason than because I can fit more of them on my shelves...