Sol Stein starts his book on writing by reminding you of something. You don't write books for yourself (at least, not if you want them to be published), you write them for readers. And readers want something from the books they read. They want to be entertained.
They might want other things as well, but ultimately readers want to be entertained, so that is the thrust of Stein's advice. How to make your chapters end suspencefully, creating tension that makes the reader want to read on. How to add colour to your novel (description, touch, taste, etc) in such a way that the reader can imagine it and wants to read on. How to pick your words and your metaphors and eliminate weak phrases, unintentional (and unnecessary) repetition of ideas, so that your story is better and the reader wants to read on.
In other words, this book teaches you some of the nitty gritty aspects of editing and reimagining writing so that your work becomes the best you can make it.
I would recommend reading it after Self-Editing for Fiction Writers by Renni Browne and Dave King, as that book has more focused points for specific editing problems. Stein on Writing covered some of the same topics, but its treatment of them was a little different, and the examples used were more numerous and often longer, which was sometimes detrimental. It's in the matter that Browne and King didn't cover that Sol Stein's book is useful. Both for pointing out the obvious (that you've missed) and in teaching you the less obvious that the other book didn't have room for.
The downside to the book was that he tried to give hints for both fiction and non-fiction, which didn't allow him to focus on either one as much as I would have liked. Separate books for each might have allowed more room to further elaborate on some of the topics.
If you want to improve your writing,fiction or non-fiction, this is a great book.