The items below are books that I have personally found useful and/or interesting as I have worked toward becoming a better writer. Some of them are real classics, while others are relatively unknown, but most should be available in an ordinary county library and ordinary bookstores. They will certainly be available through our library and through Amazon. (It's a very good idea to support your local bookstore if your town has one, and any bookstore can order any book that is still in print.)
As I look through this list, I see that most of them have been around for a very long time, most of them going through numerous editions. That's good news for you because it means your library might well have a copy. It also means you have some hope for finding a used copy. The world of English composition isn't changing very rapidly, so good advice from twenty years ago is still good advice.
You may, from time to time, be asked to prepare an annotated bibliography. The material below will show you how they should look. I have gotten the publishers' information from Amazon and used Noodletools to format the MLA citations.
Goldberg, Natalie, and Julia Cameron. Writing Down the Bones: Freeing the Writer Within. Boulder, CO, Shambhala, 2005.
Like most of the books on my list, this one has been around for a very long time. Its main contribution to the writing world is its stress on free-writing. The first edition of this book came out when the Mac computer was a brand new idea and most of us were writing on portable typewriters, but the basic thinking is still good.
Lamott, Anne. Bird by Bird: Some Instructions on Writing and Life. Random, 1994.
I have to admit that I haven't read this entire book; I'm recommending it entirely on the strength of its third chapter, "Shitty First Drafts." It's written with a lot of humor and a real understanding of how we writers actually work—and despair. The title of the book comes from Lamott's time as a restaurant reviewer for a newspaper, and refers to her first drafts, when she would go through the dinners "bird by bird" until she realized that the readers don't want to read that kind of a list. Lamott is hilarious. This is worth reading even if you aren't all that focused on the writing craft.
Rawlins, Jack, and Steve Metzger. The Writer's Way. Cengage, 2009.
This is a textbook, and it is now in its ninth edition. (I recommend you look for an older used edition; these tend to be quite expensive brand new.) His attitude toward writing is energetic and non-boring. Lots of student-written examples, most of them brilliant, and some of them downright weird. (Example: One student's memory of his grandmother's approach to killing a chicken: "You gotta wear Nike shoes because the chickens are fast and you gotta look stylish.") This would be a good book to buy at the beginning of summer, so you can work your way through it during June and July.
Strunk, William, Jr., and E. B. White. The Elements of Style. 4th ed., Pearson, 2014.
This book has been around in various editions for many years, and English majors hand it out the way some religious people hand out Bibles. White, author of Charlotte's Web and many other children's books, as well as many wonderful columns for The New Yorker, revised the book his own teacher, Strunk, used in freshman English. It's about much more than style. It's about writing effective, clean, clear English. You really need this book.
Zinsser, William. On Writing Well: The Classic Guide to Writing Nonfiction. 7th ed., HarperCollins, 2005.
The best American English is clean, clear, and direct; that's the style Zinsser promotes. A paradox is built into the writing, though: it looks easy, natural and simple, but you don't get there without hard work and purposeful editing. That's Zinsser's message.