Today I started reading A Short History of the World, by J.M. Roberts.
History is one of those things that I have a growing and occasional fascination with. I'll read a history book on a whim and I'll find the entire subject even more enlightening than the last time I studied it. I'll make a new connection or realization about the whole big-picture connectedness of the thing, or notice some similarity with current events or other past events or even fictitious events. It's one of those subjects that exemplifies the concept of growing more rewarding the more you learn about it (and not in the cheap math way of not allowing you to do certain problems until you've learned other types of math.)
It's like the more I read history the more I'm filling in blank spaces on a map. It's the same for my writing: the more I work on a particular novel or world, the more I'm painting in the empty spaces of a canvas. The beauty of the universe unfolds before me as the different pieces fall into place to reveal one glorious image.
Beyond this, reading non-fiction of almost any kind forces my mind to work in subtly different ways than it does when reading fiction. It's like stretching a different group of muscles. You may not have noticed the tension before, but once you've changed positions you realize how sore and out-of-practice you were. The amazing thing, though, is that this subtle change in my mental exercise helps the other areas of my brain grow as well.
I'm sure any long-term writer would tell you that you can't just read fiction. Yes, reading fiction is important for a variety of reasons (stimulates the imagination, inspires you with technique, informs you as to the market, etc.) but that's not going to get you very far. Variety is important. Like a well-balanced meal. Adding some non-fiction to your reading diet brings a whole new wealth of inspiration and information that can be applied to writing. If only for examining the different ways that other styles of writing use words and create meaning, reading non-fiction (or, quite simply, changing your reading habits somewhat) intently can have huge unforseen benefits for your writing.
I should know. I can't count the number of breakthroughs I've had on stories that I thought had become stale by reading through a non-fiction book. Or the number of stories that I've had pop into my mind while reading some history.
Reading outside your comfort zone (like doing anything outside your comfort zone) allows your well-worn mental paths to rest, enabling them to develop ideas sub-consciously, leading to a greater level of inspiration than you could have designed.
At least in my experience. :)