Stephen King's On Writing has some great writing tips as well as a look into a writer's life. Even if you aren't into horror, read this book. It can be applied to any genre.
I looked into The Faith of a Writer: Life, Craft, Art by Joyce Carol Oates because I wanted to know more about how a writer's mind worked to see if I was the only one thinking this way. This book, like On Writing, gave me the peek and the reassurance I was looking for.
If you write horror, you need this book. Each genre has characteristics. On Writing Horror discusses all the traits of a horror story in detail. It also has a list of books all horror writers should read and information about subgenres and marketing. It is the most extensive book about writing horror.
I discovered no matter how many times I reread my story, I still missed some grammatical errors. The Artful Edit by Susan Bell taught me different editing techniques- different ways to gain distance from my writing. One advice given I use often. I change the font to something totally different. It helps me look at the work differently.
Writer's Guide to Character Traits is extensive and I highly recommend it. It gives you detailed description of personality types of adults, children and criminal types to name a few. This is not something that should be used by itself nor should you take each personality type and just apply it to one character. I already had a character's personality in mind so I used this book to see certain behaviors I may have missed. But this book can help you flesh out your villain.
Characters Emotion and Viewpoint helps you create a fully fleshed out and believable character. It discusses the trait of characters found in different genres, how to show emotion through dialogue and discusses the three points-of-views- First, Third and Omniscient.
Bibliophile's Dictionary so much. It's a thesaurus arranged by subject and contains usual but useful words not found in other thesauri.
Dictionary.com a lot but sometimes it's easier to have a thesaurus on hand. I recommend The Synonym Finder.
The Complete Gun Owner: Your guide to selection, use, safety and self-defense is a good starting point. It helped me focus my research.
Get Known before the Book Deal is about building a platform. It discusses, among other things, determining and connecting to your audience, developing your niche and finding your style.