Checklist for Plain Language
plainlanguage.gov
plainlanguage.gov
When you’re reading something from an institution that seems unnecessarily obscure or indirect or passive, there is usually a reason, and that reason is usually that if they had stated the plain truth, it would make them look bad.
There was a great geeky essay about the language of the video game portal that went into this:
https://www.gamasutra.com/view/news/114904/Analysis_Portal_a...
And of course the classic Orwell essay:
http://www.orwell.ru/library/essays/politics/english/e_polit...
https://en.wikipedia.org/wiki/Rachel_Carson#List_of_works
But as for writing advice, the CIA isn't so bad.
Thinking and Writing: Cognitive Science and Intelligence Analysis
https://www.cia.gov/library/center-for-the-study-of-intellig...
Also, the earlier Bestiary of Intelligence Writing.
https://www.cia.gov/library/readingroom/docs/DOC_0000619161....
https://www.england.nhs.uk/ourwork/accessibleinfo/
(I have a few problems with the NHS England accessible info stuff, most of which are around how inaccessible it all is.)
https://www.gov.uk/guidance/content-design/writing-for-gov-u...
https://www.ftadviser.com/pensions/2017/09/25/bt-goes-to-cou...
This switch to CPI was bared a few years before for British coal - the two schemes of arrangement differ in a very minor way - that a lay person would say where identical
Strunk & White contains some timeless advice. Most (but not all) of the habits it identifies as bad writing are still considered bad. But for positive examples of how to write well, you can't follow the example of a book from 1918. Its language is inherently dated and it predates linguistics as a science. You need a newer style guide.