Just as I was once shocked to learn that not everyone has huge bookshelves in their house, I am sometimes surprised to learn that many readers don't read nonfiction, except for school. (I'm sure there are also many nonfiction readers who don't read fiction, only I don't know them because I dwell in the fiction world and they're probably not reading my blog.)
I LOVE LOVE LOVE non-fiction. Always have. My childhood library, mind you, was a wee thing. It was built within my readerly lifetime and nowadays the small building is bursting at the seams with books, but when I was a kid, it was new and there was plenty of room. There was one stubby aisle of children's fiction. Children's non-fiction was lumped in with adult non-fiction, and that section was more like seven aisles, so it became my habit to wander down every aisle of children's fiction and the non-fiction every time I went to the library and pick up whatever caught my eye.
I started out as more of a fiction reader and started moving more toward non-fiction in my early twenties, and then back a bit more to fiction again when I became serious about fiction writing. My favorite books ever are fiction, but my home library is 75% non-fiction.
My favorite non-fiction books have always been about history or other countries, particularly about some aspects of people's every day lives. This has shaped who I am as a person and a writer. I haven't been to another continent, and I grew up in a pretty small world. I didn't realize as a kid I was learning to think in global terms and notice patterns throughout history because of the many many books I read. In the past five years I've read about how airplanes work, the history of fairy tales and children's books, archaeology, the Age of Enlightenment, manners around the world, blindness, Romantic poets, Chinese foot-binding, immigrants and Ellis Island, the Masai people, the Chicago Worlds' Fair, early European explorers in the Americas, remote tribes in South America and Asia, three generations of a Japanese business family, the Mafia, conjoined twins, the Hindenburg, and the history of food; housework; Publix grocery stores and Woolworth's. I've traveled, via book, to Japan, Victorian England, 18th-century France and Italy, New York City, India, Russia, Mongolia, and Maine, and I've learned a little more about Isaac Asimov, Helen Keller, Sylvia Plath, Tasha Tudor, Louisa May Alcott, Colette, Frederick the Great, Charles and Emma Darwin, Lord Byron, Madame Pompadour, L. Frank Baum, the Romanovs, Horatio Nelson, the Mitfords, Frida Kahlo, Josephine Baker, eternal favorites L. M. Montgomery, Laura Ingalls Wilder, Jane Austen, and the Brontes.
For fantasy writers especially, I think it's good to read non-fiction...whether you're interested in animals, food, rock formations, castles... You can bet you'll use it for world-building somewhere. Fiction is great, and I read tons of it, but non-fiction lends creativity and authenticity to fiction, and I don't just mean the reading you picked out for targeted research. Sometimes it's the random book you picked up on the fly that turns out to be just the thing.