When writers write and when readers read, they often explain their love of stories by saying that the words take them away to other times and places. Their imagination is fueled by stories and, in the case of writers, finds an outlet in stories. While it is true that very small and seemingly inconsequential things in daily life can inspire a novel, books and the written word continue to be the prime medium for the activity of the mind; reading promotes some degree of intelligence.
To have a well-rounded mind, however, it is necessary to not focus on a single genre of literature. You shouldn't read only fantasies and fairy tales; you shouldn't even have a steady diet that is 53% fantasy and fairy tales. You shouldn't have a steady diet of romances, "Christian" or secular. Nor should you wear a track from the library door to the historical novels. Histories should not gather dust while fiction is being constantly pulled off the shelf. There is no genre that can be indulged in to the exclusion of all others; the mind will be stunted if fed the same thing day after day, just as the body would if you only ever ate carrots or potato chips.
Oddly enough, one of the bits of advice most promoted by many writers today is that you should read extensively...in the genre of your choice. If you want to write historical fiction, read historical fiction. If you want to write fantasy, read fantasy. Never mind that this may very well mean that your plots, characters, story arcs, and what-have-you are being fed to you by authors who have come before, or that you are stuck in the rut of one genre both as the source and outlet of your imagination. 'Read in one genre, write in one genre' is the rule of the day, and so authors are pigeonholed into specific fields of writing to develop themselves there until they are ready to expand (if not forever).
This is not wholly ridiculous. From a marketing standpoint, it is true that if you write and publish a historical fiction, then write a fantasy and want it published, you will likely have to seek out a new publisher. I speak from personal experience; I am currently querying Wordcrafter, a fantasy, and it is almost like being an entirely new author. I have no guarantee of acceptance. But I wouldn't trade the time I spent writing that story for the certainty of publication, and I would far rather have been writing Justin's story than grinding out another historical fiction. Of course, I am writing a historical fiction now. I simply didn't want to then, because I didn't have the inspiration for one.
Writing is an art, although it must be balanced with the more "practical" side of marketing, and some of the most renowned artists are those who experimented in many different mediums. Michelangelo was a sculptor as well as a painter and architect; Leonardo da Vinci dabbled in a dozen things, from sketching to painting, from writing to inventing. In the realm of writing, Agatha Christie is most famous for her mysteries, but she also wrote romances. C.S. Lewis wrote essays on faith and philosophy as well as fantasies and "science fiction." Rosemary Sutcliff, acclaimed for her Romano-British works, wrote children's books, stories set in the Middle Ages, some nonfiction, and retellings of The Iliad and The Odyssey.
These artists were never equally lauded in all mediums, but that was not their purpose; their imagination was fired in many different directions, and so they followed that and did not remain inside the box of their own especial genre. Their minds were well-rounded, so that they could and did tackle fantasy as eagerly as nonfiction, sculpting as readily as painting. Practically speaking, if you read widely, it is unlikely that you will be able to stop your mind from developing tales in many different genres - and this is not a bad thing, even if you are not as skilled with one as with another. It's good for the imagination to expand, and not to be allowed to stagnate in a single medium.