That's not true at all. There's wide variation on the quality of writers even if you restrict yourself only to those who can spell and use correct grammar. Things like word choice, omitting adverbs and filler, sentence length, organization into paragraphs, structure of the piece, transitions, use of formatting, and supporting details all make for noticeable differences in how easy it is to read.
Saying "there's no way to get better" is like saying "once you learn the syntax of Java, you know everything there is to know about programming." It just belies your ignorance of how much else there is to know.
I said there is no good way to get better, by which I mean a way that you can objectively measure whether what you are doing today is better than what you were doing yesterday and most crucially of all: what you need to improve.
You can do that with programming since each choice you make gives a tradeof in terms of speed, complexity, etc. I haven't seen a book with a similar set of concerns with regards to writing.
If you define "good" as "can be objectively measured", you've cut out most of the interesting problems. Data is a tool, not a crutch.
If you write UI software, how do you know that you're getting better? You can use A/B testing, sure; but how do you decide which metrics to use? Ultimately, it comes down to taste: you look at one possible approach, you look at the other, you give them both an honest appraisal, and then you say "I like the first version better."
It's the same thing with writing. Write a sentence. Now rewrite it, removing superfluous words or changing the phrasing or breaking it in two. Which one did you like better? What's different about it? That's how you improve.
As for their being no books - pish posh, there're plenty of books on improving your writing skills. The Elements of Fiction Writing is one such series. I've heard Steven King's On Writing is also quite good. The best way, however, is to sit down with an experienced writer and have them critique your work.
Failing that, you could always go look at Paul Graham's Etherpad recordings of his writing process and see what he does.
>> that you can objectively measure whether what you are doing today is better than what you were doing yesterday and most crucially of all: what you need to improve.
It is hard to judge your own work, The best way is to get a friend to do it.
Another approach it to re-read your writing and compare it to early work. Letting 6 months go by will allow you to re-read your work with fresh perspective. Compare to what you wrote 2 years ago.
The cheapest approach is to take a break for a little while, then come back and re-read what you just wrote. Is it well-organized? Is anything phrased awkwardly? Are there parts you can tighten up without losing anything?
Just taking the time to do this can make a huge difference in the quality of your writing.
Saying "there's no way to get better" is like saying "once you learn the syntax of Java, you know everything there is to know about programming." It just belies your ignorance of how much else there is to know.