Plain English has been a 'super power' for my report writing ever since it was drummed into me by a former CEO and MD at a finance and economic consultancy I worked at after University.

"Style toward Clarity and Grace" by Joseph M. Williams is, imho, the most thorough, yet comprehensible, guide to how to write in Plain English. It goes beyond short 'do this, don't do that' guides by explaining why, and how, English can be unclear with exercises to help transform your writing.

I can't stress enough that the ability to take both simple ideas, and complex ideas, and write both in a clear and compelling way that inspires action, will pay dividends over any career.