Perhaps the most useful guide to writing I have ever had was the government's Plain Language guidelines. In the 1990s, Bill Clinton issued a memo stating that people should write things better in the government because no one could understand what was being said between agencies. Inside the agencies, specialized vocabularies had developed that no one outside understood.
I've recently started to review these guidelines, and man, I was in desperate need of review. Clients have been asking me WTF I meant in my responses to them, when I thought it was a clear as day. Come to find out, I haven't been COMMUNICATING. The thing I think I do the best. Heh. Woopsie!
Anyway, if you write anything, the ideas in this manual are excellent. The things I do that you like about my writing are within the guidelines. The things that don't make sense are the places where I've gone outside the guidelines. If your goal is to convey information to people, this book helps a lot.
And it's free. Can't beat FREE.