While persuasive writing could be seen as forking in two extreme branches, ads and long-form copywriting, that are aimed at selling something, the art of sales by means of content and copy can be reduced to a lesser denominator that is neither a long promotion or an advert.
To get what I'm saying, think of forms of long content advertising, reduced to three or four sentences or paragraphs. I'm talking of copy not bigger than, say, four hundred words.
Sales copy as I want to classify it, is not a glorified advertisement either. Because on the internet, there are a hundred and one interstices where sales copy must or could be implemented.
The germane approach at selling can be extremely convenient for the buyer. If a glossy approach can't be implemented due to obstacles and challenges posed by the medium, budget or other problems at least a moderate approach at able sales copy must be in place anyway.
I call your attention to this because when you are a customer, there's nothing as off-putting as a catalog that wasn't correctly fleshed out with copywriting sales micro-copy, to give just an example of places where it's a waste not to use it. A catalog without descriptions is just a catalog in name, because a crucial element is lacking.
This is a salesman's book on selling and salesmanship. This classic sales book was originally published in 1937, it stopped being printed in 1954. It’s old, still, its teachings on the methodology of selling are elementary logical mechanics one has to learn to be a good salesperson.