Most of the time, though not always, people tend to write either non-fiction or fiction. Every week in my local paper, there are "top ten" lists separating books by these two categories. However, I'm always amazed at how much research goes into writing fiction. Have you ever read a really great historical fiction novel and been in awe of how much they had to learn about the time period before they wrote the book? I've read author interviews where they spent years--years--researching before the actual writing part took place.
As a total research and science nerd who geeks out over NatGeo and the Discovery channel, I loved doing research for my book. I had some great conversations with professors at a respected astrophysics department and learned a ton. One of them told me I inspired him to write a new question on his graduate student exam. The great part about using non-fiction in fiction is that you can get creative in how you implement it. You have more leeway than you do with non-fiction, plus the obvious--you can't have vampire space monkeys in non-fiction and they're plain fun.
What about you? Did you enjoy the research part of your book? Anyone who has written non-fiction and fiction?