Generally, I don't spend my days poring over books about writing, and chances are, neither do you. Most people write on instinct, having a vague idea of their characters, their setting, and a basic beginning, middle and end (if they're lucky).
Over the years, as I've immersed myself in the company of fellow writers, both accomplished and amateur, I've discovered that pretty much everyone reads or owns the same handful of manuals to help guide your (fiction) writer's journey. So here are the Big 3:
Best known for his cameo in the Nicholas Cage film Adaptation, Bob McKee's Story goes into the structure of a story whether it's a book or screenplay, and is quintessential reading for the fiction writer. I really don't know what to tell you about this book except that every now and again, I pick it up and it succinctly articulates the process of storytelling and never fails to get my gears going after they've ground to a halt on some plot point.
The Writer's Journey: Mythic Structure for Writers by Christopher Vogler
Based on The Hero With A Thousand Faces by Joseph Campbell, this much more concise book outlines the basic steps all heroes must take in a story. It touches on all the common elements in a story, from the Call to Adventure to Returning with the Elixir. It's likely you'll remember some of the lessons in this book from a high school English class. Now you can finally put those lessons to good use.
Elements of Style by William Strunk & E. B. White
Everyone calls this the writing bible, and there's a reason for it. It's small, easy to read, and really simple to understand. It focuses on the mechanics of good writing...but you're probably already nearly there, right?
There are dozens of other titles people will recommend to you: Stephen King's On Writing has been recommended to me numerous times; any number of writing guides for different genres will also build you library as you explore different genres and subgenres.
I'd recommend borrowing titles from your local library to test them out--some books are so dense with jargon you get lost in the craft; others are just plain useless. But since everyone's different, and we can't all afford to just buy every writing book out there, find your happy medium and build your personal library slowly.