There are two books I use every single time I sit down to write. They are super helpful and, if you don't own them, I would recommend ordering them online or visiting your local book merchant. The first of the two is On Writing: A Memoir of the Craft by Stephen King. This book is wonderful because it offers an in depth look at King's own start with writing, carrying you through his young adult life, and into his time as an accomplished author. He essentially explains how he made it, but the story is inspiring and written wonderfully as you'd come to expect from the master himself. The second part of the book is more structured like a textbook, however, don't let that turn you off. The knowledge he passes through his words is presented in a manner that feels friendly and conversational, unlike any English or writing courses you may have taken in college. Get the book.
The other book that is invaluable is The Elements of Style by William Strunk Jr. and E.B. White. I happen to have the Fourth Edition, but earlier editions don't necessarily have White involved with them. This book is definitely more the style of a textbook and doesn't offer personal experience as King's does, but the assistance it provides is something every aspiring writer needs to learn, or, at the very least, refresh themselves with. This one has everything, from where to use commas, to commonly mistaken phrases. It guides you away from mistakes you may not even realize you're making and helps you stay "clean", that is, put forward the best work you can. This one is a must.
One quote I picked out from The Elements of Style was:
"Writing is one way to go about thinking, and the practice and habit of writing not only drain the mind but supply it, too."
Love it.
Barry