Most guidance you read on writing is aimed at the writer in terms of how to structure the work, develop the characters, refine the tone of voice, ad nauseam, i.e. how to improve your craft from a writerly point of view.
I, from an advertising background, constantly think of the reader.
'Of course,' you cry, 'it's obvious you have to think of the reader when you're writing an ad; you are trying to sell them something! I am trying to write a book and not flog someone a dead horse.'
The fact is all your efforts can amount to flogging a dead horse if you don't think of your reader. Because the fact of the matter is you are trying to sell them something. Your book.
There is an old adage in advertising that states you can sell anyone anything once; the trick is to be able to sell them it twice. That is where the money lies.
If you write a tome thinking only of what appeals to you, without doubt you will be the first to cry when you receive nothing but rejections. The whole function of publishing is geared towards thinking what the market, your potential reader, wants to read. If you are not geared to the same degree, there will be a clash of gears.
If, by some oversight of an editor's judgment, your book makes the shelves, people will buy it. A few, nonetheless some. However, if it fails to live up to their expectations those few will never buy another book you produce. As a reader, you know how many books you have bought on the spur and left, half-read, to curl at the edges never to buy a book by that author again. Worse than that, you will not be the beneficiary that no amount of advertising pounds/dollars/yen/yuan/rupees can buy; recommendation by word of mouth.
It is true advertisers chuck a lot of money at the reader, or market, to discover what the reader thinks, eats, drinks, wears and the quality of air that he or she breathes, so the copywriter has a fair idea of the person they are appealing to. The poor author does not have these resources. I correct myself: the author has an immeasurable wealth of data just down the road at their nearest bookstore.
Bookstores are filled with books that have succeeded in every describable genre. Read them. Have your favourite open on your desk as you write. These authors have an insight into what your readership wants. Do not be afraid of being a clone of the person you admire; you are far too surly, feisty, cocksure, a pain-in-the-arse to be anyone other than yourself and it will show in your writing. As I used to say to my girls, 'Don't copy my mistakes, learn from mine and find your own.' (They now tell me the same.)
Did Tolstoy, Dickens, Zola, Twain, Garcia Marquez, and others from different continents and sub-continents advocate what I am saying. I doubt it. The understanding of their readership was in their DNA; they had no need of trite observations from some hack lowly as I. For the geniuses among you, I apologise; for those, like I, struggling, I hope my comments help.