In order to explain the main purpose of this book, I need to talk about style and voice.
Style is about the choices you make between valid alternatives. For example, do you use parentheses, dashes, or commas to set off a clause that "interrupts" the main sentence, or do you not put such clauses in at all? All of these are valid choices, and if you make one particular choice consistently, it becomes part of your style.
Voice is largely, but not completely, made up of the sum of all your style choices. It also takes in point of view and a few less definable qualities. This is what tells us that we are reading Ursula K. Le Guin rather than Neil Gaiman, however similar their writing might be in some ways.
Voice is also something that belongs to characters, as well as authors, and one of the skills that raises authors above mediocrity is the ability to give their characters distinctive voices. Again, point of view, the things they notice and talk about, has a lot to do with it, but so do stylistic choices. I often make my characters sound less like me by having them choose a word which wasn't the one I first thought of.
George Orwell, in a famous essay, spoke about language which, like clear glass, was there for people to look through, and other kinds of language which were like stained glass: designed to be looked at. In fact, there's a spectrum of style, which Jeff Vandermeer sums up well in Wonderbook. (I recommend that book, by the way, which is about everything about writing that I don't cover here: how to find ideas and develop them into stories.)
Vandermeer mentions four levels of style, which, as I'm sure he'd be the first to acknowledge, are points on a spectrum rather than distinct steps.
Minimal or stark style has hardly any detail or description, leaving a lot to the reader's imagination. Done badly, it's dull and unengaging, emotionally distant and ploddingly literal.
Invisible or "normal" style aims to disappear, like Orwell's clear glass window. It's the most common style of writing, particularly for commercial fiction. Although it doesn't sound the way people actually talk, because we're used to fictional conventions we think of it as sounding like the voices of real, ordinary people. Done badly, it's mediocre, bland and lacklustre.
Muscular or conspicuous style uses more sensory detail, more metaphors, and more complex sentences than ordinary people tend to in real life. It draws a little attention to itself. Done badly, it seems too clever and gets in the way of the story.
Lush or ornate style draws a lot of attention to itself, like a beautiful woman dancing in a sequinned dress. Extended metaphors, long sentences, passages of pure description, language tricks borrowed from poetry, unusual word choices and a layering of detail give an overall richness to the prose. Done badly, it's like wading through treacle, and produces impatience in the reader who just wants to know what happened next.
The commonest style, and the one that's easiest to read and easiest to write, is "invisible" style. My advice in this book is aimed at helping you to write invisibly, because if you use the basic tools of language correctly, nobody will notice that you're using them. Using them badly draws attention to your writing, for all the wrong reasons. It's like you're a cabinetmaker, and you've left tool marks on all your surfaces and your joints don't fit well. People will notice that you're incompetent with your tools, rather than noticing the piece of furniture and forgetting that it was made with tools at all.
Myself, I wouldn't advise attempting anything other than invisible style until you can write invisible style competently and smoothly. I see far too many people trying to write like authors they admire without the basic grasp of language and style that those authors possess. In particular, if your vocabulary isn't as large as you think it is (and most people's isn't), you'll embarrass yourself by using fancy words that you don't really know the meaning of. I see this all the time in books I review. If you're going to use a fancy word, look it up and make sure it means what you think it means.
The more you depart from normal or invisible style, the more obvious any mistakes will be, because the other styles deliberately call attention to your use of language. If you attempt a lush style, in particular, without understanding exactly what you're doing, it will fail, and fail horribly. A lot of readers don't like lush style much even when it's done well (I'm one of them), and seeing it done badly is agonising.
The time to let your prose draw attention to itself by dancing is after you're confident that it won't draw attention to itself by falling flat on its face.
By the way, if you want to read a wonderful (though tragic and heartwrenching) example of the use of changes in style and voice to convey a character's experience of the world, find a copy of Daniel Keyes' Flowers for Algernon (either the original short story or the novel that he later expanded it into). It's the journal of an intellectually disabled man who gets an experimental treatment that increases his intelligence to well above average, and what the author does with language is brilliant. The impact of the story would be much less without it.
It's also a great example of knowing when to break the rules. All rules--even the ones for correct punctuation--can be broken to create a deliberate effect (such as conveying the writing of a semi-literate, intellectually disabled man). Some people will tell you not to worry about the rules, that telling a good story is the important thing. Telling a good story is extremely important, but another important thing is to know the rules, and know the effect that you'll get from following them, and the effect that you'll get from not following them, and choose which effect you want--rather than accidentally create the effect of looking unprofessional.