When I'm doing freelance writing, I have two books handy. One is the The AP Stylebook. The other is Strunk & White's The Elements of Style. With the work I do for DMS, these books are valuable; I refer to them frequently.
Of course, that's my freelance life. I use different resources in my fiction life. Before starting the character biographies and outlines for the coffee house book, I (re)read The Things They Carried by Tim O'Brien and As I Lay Dying by William Faulkner. Both had elements I wanted to be more familiar with for the purposes of my storytelling. As I'm completing the character biographies, I have a copy of The Gift of Stories: Practical and Spiritual Applications of Autobiography, Life Stories, and Personal Mythmaking by Robert Atkinson. I'm also perusing shelves here and there to find some good resources about gourmet coffee.
When I started writing fiction seriously, I didn't realize how much I would lean on nonfiction resources. I naively thought I would sit at a computer and make up a story off the top of my head and that would be that. Now I do a lot of research and writing (character biographies, outlines, backstory) before I even start telling a story.
The right resources can really push a story in the right direction. They can create a depth you didn't know could be there, a plot twist no one saw coming, or even clarification on something you thought you knew well enough to write without fact-checking. Regardless of the reason, finding good resources is an important part of writing fiction (and nonfiction). And every writer has books they keep handy when they sit down to write.
What are your writing resources? How did you come to use them?