There are many resources out there for writers, each looking at a different aspect of writing. Some will go into the nitty-gritty about how to structure a story or build a character. Others will go into narrative voice and points of view. And of course, you have the countless number of resources about punctuation and grammar.
Then you have all the genre specific materials that you might pick up along the way. For me, these include books about weaponry, criminal investigations, herbology, and, probably the most odd edition on my shelf, improvised munitions (complete with actual, functional recipes—thank you, U.S. Army).
If you're anything like me, most of the writing resources you have sitting on your shelf are books that you refer to occasionally. They're useful to have, but they aren't something that are an instant GO-TO for most things you write. Perhaps you've read them through cover to cover once, but they tend to sit on the shelf collecting dust.
But, there will be those books that you just can't do without. You love them so much that you seem to have accumulated multiple copies of them along the way. They are books that are so well loved that your paper copies (if you have paper copies of them) have little tabs added everywhere, marking important pages that you keep going back to. Or the electronic copies are annotated to the point of exhaustion.
And if someone asks to borrow that book... NO WAY! Get your own.
The following list of books is like that for me. These books are volumes that live within arms reach of my desk. For a few of them, I have both paper and digital formats, that way I can always have them with me when I'm writing. Regardless of the fiction genre you write, these books are beyond valuable.