To improve writing without a teacher, then there really is only one feasible option: read Chinese books, etc., and adopt their writing into your skill set. (See: Is it true that improving the reading skill also enhances the writing skill and improving the listening skill strengthens the speaking skill? at LanguageLearning.SE.)

Without a teacher, progress is surely going to be significantly slower. (Part of my motivation to get a teacher was the realization that native Chinese speakers have spent considerable hours with Chinese teachers (many more hours than me). The other part was Nankai students going from nothing up to my level very quickly.)

Study methods vary with one's goals. E.g. wanting to pass a HSK test vs. wanting to use Chinese for conversations. With respect to writing...

To improve writing, we need a way to get feedback; a book cannot do this. Simple sentences are feasible to get right most of the time, but more sophisticated sentences is challenging:

How do you know if a metaphor works in Chinese? (My experience is that if I take an instantly recognizable metaphor in English, and translate it into Chinese, nobody "gets it".)

How do you know if there's a better choice of words or sentence structure?

How do you know if an idiom is recognizable? (E.g. I recall using 有嘴没舌 and 一字不改, and my teacher couldn't figure them out.)

I sometimes ask questions here using the writing-critique tag, but I can't just flood the site asking about every single sentence in Chinese I write.

In any case, many Chinese textbooks are listed at Chinese textbook grammar index at the Chinese Grammar Wiki (but they're much cheaper on e.g. Jingdong).

The HSK Standard Course books are incredibly boring, but are good for passing HSK tests.

I'm currently going through 发展汉语-高级口语 with my teacher; it's challenging, but more interesting.

Actually there is one technique that might help, which I asked about on LanguageLearning.SE: Translating paragraphs then comparing with an existing translation: is this technique used to improve writing skills?. The idea is to

It's not going to point out errors, but it might help indicate smoother ways of writing (e.g. better word choice and sentence structure).

(See also How to improve writing fluency at LanguageLearning.SE.)