For starters, I would say Enterprise Content Strategy: A Project Guide is a good choice. Though really, the important take away is to know your target audience. There are a few other gems over on xmlpress.net that are also worth a look.

Next, read the sort of things you want to write, then try writing the things you want to write. It's really that simple!

- Minto's Pyramid Principle (writing as structure; state your point and then unfold more and more details as you go)

- Clear and Simple as the Truth (writing in the "classic style")

- There is no dearth of books about writing, with many recommendation threads on HN already.

- Research the mental model of the "rhetorical move". You will find that there are several books on using patterns in writing. Many are written for non-native speakers of English engaging in academic writing. But the idea is very useful for all genres!

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You didn't ask for them, but here are some favorite tips:

Think about your writing structurally / architecturally. Be mindful of patterns. Strive for clarity and concision. Know and consider your audience.

Read. Question what you read. What qualities does the writing have? Why? What structures and patterns does the writing have? What are the functions of the parts and the whole? Why and how do the patterns and functions work well?

Question your own drafts. What qualities do you want your writing to have? Why? What is the essence of those qualities? Have you achieved them? Can you design exercises for yourself to develop those qualities?

Write every day. Engage in deliberate practice of certain aspects of writing.

Write and revise several drafts. Let drafts incubate.

Try writing by hand. Try writing on notecards and rearranging them in piles or on your physical desktop. Try dictating and transcribing. Try typing each sentence on its own line (it makes rearrangement easier by moving lines up or down with keyboard shortcuts in a text editor like Sublime Text). Try thinking through and mindmapping your structure before writing the draft. Try drafting in a good outliner, such as Bike or OmniOutliner.

Develop and maintain a professional relationship with a good human editor.