The Chicago Manual of Style is far and away the most widely used book in publishing, but writers really don't need it. Neither do editors, when it comes to grammar and ordinary style questions. Editors already know grammar and ordinary style inside out, and don't have to look it up in any manual. And, in fact, many editors simply don't follow Chicago where these are concerned. (THough, heaven help us, America is getting a terrible reputation where editors are concerned. Conglomerates tend to hire based on how cheaply they can get a new editor, rather than on how knowledgeable that editor is.)
Chicago is used because publishers publish a huge variety of books, and this means needing a standard for things other than grammar and ordinary style. Headers or all sorts, how to insert quotes or letters, etc., all play a part in putting a book together. So do photos, kerning, and a dozen other things.
Chicago is a great book, if you're dealing with more than grammar and style, where it gets terribly wishy-washy and politically correct in newer editions, but it's not the best choice for a writer who simply wants grammar and style.
At one time, Words Into Type was the best and most used book, but when it stopped being updated, Chicago took over. But it's really far better for publishers than for writers.