The answer to this question is simple. Fiction writers read everything they can get their hands on.
Of course they read the kinds of things they'd like to write. Romance writers read romance, mystery writers read mysteries, etc. It's sort of a requirement of writing in a genre. If you don't read in that genre, how do you know how to write for it?
Fiction writers also read fiction in other genres. Let's face it, no one wants to read the same thing all the time. We need that spice of life, variety. Plus it's good to keep up on market trends in other genres. Who knows when you'll find something you can incorporate into your own work?
Fiction writers read books, articles, etc. from the publishers they'd like to get a contract from. It's called market research. You need to know what that particular publisher is looking for. What type of things do they publish? Does it fit with what you write? For example, you wouldn't send a collection of poems to a company that only publishes "How To" books. Is your manuscript too much like another title they've recently published? You may want to submit it to a different company.
Writers read about writing. Good writers always strive for improvement and there is always something more to learn no matter how long you've been perfecting your craft. The number of books written on this subject should be proof of that. And then think of all the magazines, web sites and blogs on writing.
Writers read non-fiction. They read about subjects they need to research for their fiction writing. They read nonfiction just because a certain subject interests them. Plus you never know when you might find something you can use in a story.
How about the Bible? I'd say that's a given for White Rose authors, at least.
Fiction writers read magazines, books, newspapers, blogs, and even cereal boxes. The size of the piece is not important if it contains something of interest to them.
So there you have it. Add it all up and you come out with the fact that writers will read anything and everything they can get their hands on.
So, what have you read lately?