There are some books that are basic and fundamental. These are must haves for your shelf.
A decent dictionary. It's nice to have one in an actual book form, but you can get by with http://www.dictionary.com/.
A thesaurus. Otherwise you will end up using the same five adjectives over and over again. http://www.thesaurus.com/ is ok in a pinch but I find my copy of Roget's more thought-provoking.
Collins Good Writing Guide. It covers grammar, punctuation, spelling, confusable words, and tips for better writing. Very useful and engaging.
Fowler's Modern English Usage. Full of those words that we think we know how to use but are not sure we're doing it quite correctly. If you've ever had to sit and think about the difference between affect and effect, or whether what's in your mind is bare or a bear, then you need this.
A good guide to the basics of grammar and punctuation is useful. I use the Oxford A-Z of Grammar & Punctuation and the fabulous Larry Trask's guide. I also follow the Daily Grammar Blog.
How To Write Books
There are innumerable books out there on how to write. I've read a lot but I think I've barely cracked the surface. These are the ones that, so far, I've found the most useful.
Novelist's Essential Guide to Crafting Scenes by Raymond Obstfeld
This is a close look at creating scenes and how to use them to best effect. My style relies heavily on scenes; I like action and dialogue and to show things happening on the page. This book has helped me refine that.
Novelist's Essential Guide to Creating Plot by J. Madison Davis
There are lots of books about plot and there are few that I have connected with. This was very practical, especially the section dealing with handling multiple plot lines.
How to Write Damn Good Fiction by James N. Frey
An excellent practical book. And the last two chapters, Seven Deadly Mistakes and Writing with Passion, are especially inspiring. Frey believes writing is important and that's good to remember.
Writing Fantasy & Science Fiction by Lisa Tuttle
Writing SFF is not different from writing anything else, but you rarely find examples from the genres in how to write books. This book is clearly written in easily digestible chunks and takes apart the stuff you think you know. You'll learn something.
How to Write Science Fiction & Fantasy by Orson Scott Card
Half the book is about writing SFF and has some good stuff regarding language. The other half is about being a writer and how to get the most out of critique groups and conferences.
Self-Editing for Fiction Writers by Rennie Browne and Dave King
This is excellent. The chapter on internal monologue helped me get to grips with something I had found difficult. It made clear to me what internal monologue is and how to do it well.
The 38 Most Common Fiction Writing Mistakes by Jack M. Bickham
Sometimes an example of what not to do can be more instructive than examples of good writing. Particularly when you recognise that you do some of those things.
Word Painting by Rebecca McClanahan
This is about doing description well. Very detailed, with lots of examples.
The Art of Subtext by Charles Baxter
This is literary criticism rather than a how to write book, and is in danger of causing brainache. Nevertheless, if you're interested in complexity in fiction this is worth a read.
Write Great Fiction: Dialogue by Gloria Kempton
Years ago, I got some feedback that my dialogue was a bit wooden and samey. So I read this book and learnt how to do it better. Recent feedback indicates that it's worked.
Write Great Fiction: Revision & Self-Editing by James Scott Bell
Very helpful in fixing up a first draft. It inspired the spreadsheet I'm using for my work-in-progress.
Elements of Fiction Writing: Beginnings, Middles & Ends by Nancy Kress
Excellent on structure and lots of practical advice for strengthening your work.
This list represents about half of what's on my bookshelf. The rest I've found less helpful and wouldn't recommend them but are still useful enough to keep. I've given away even more.