Post History

I think you need to distinguish four aspects of writing and focus on the ones that you most want to improve on. They are:

* The mechanics of writing: Sentence structure, punctuation, etc. For this books and/or classes on grammar and composition may be useful. 

* The craft of writing: This has to do with the mechanics of what a story is, how to create and sustain the interest of readers, etc. There are lots of books and courses available on this. Some courses focus on the craft of writing, and some focus on the mechanics or the art, so if you take one, make sure you get the one you are most interested in. The books and the courses will teach you the basic structures of story and give you a vocabulary for talking about them. (The vocabulary of the hero's journey is particularly valuable in this regard.) Once you have that, you can begin to read with attention, meaning that you look for how the writers you read are using the elements of craft and structure as they build their books. 

* The art of writing: This has to do with the beauty and the depth of the works you create. Art usually exists on top of the structure created by craft, though some experimental works depart from it (and are seldom read as a consequence). If you take an MFA, you are likely to find a focus on art, as opposed to the focus on craft you would expect to get from something like Writer's Digest. But art cannot really be taught the way craft can. Art is largely a matter of vision, and while vision can be encouraged and fostered by exposure to and discussion of great art, there is no process for vision and therefore no process for art. You don't so much learn art as develop it. But you don't need art to be a successful commercial author. Plenty of authors make good livings, or even millions, on craft alone. 

* The business of writing: Publishing is a commercial enterprise. It exists to make money for shareholders by selling books, and it focuses on finding and producing the types of books that it can sell at a profit. This means that it looks for books that address certain specific market segments, which, in publishing, are defined by genres. Each genre had a certain set of expectations that readers bring to the books they buy, and certain constraints about length, language, spiciness, etc. Genres and sub-genres and certain themes and subjects within genres go in an out of fashion, so a book that made money last year may not make money next year. Trying to get the industry to accept something that is not addressing current genre tastes, while not impossible, can be very difficult. To learn about the publishing business, look for books and websites on the business side of writing, but also be aware that the business side is in constant flux, so you can't just learn it once and forget it, you have to keep up. 

But beyond all that there are the basics: Read a lot. Read with attention. Write a lot. Write with intention. Get as much dispassionate feedback as you can.