One of my absolute favorite books on writing, The Elements of Style, by William Strunk jr., is free if you read it as an ebook. It’s copyright has expired.
“This style manual offers practical advice on improving writing skills. Throughout, the emphasis is on promoting a plain English style. This little book can help you communicate more effectively by showing you how to enliven your sentences.”
It’s not very long and it’s packed with good advice. A lot of it valuable for writing in any language and some of it more skewed towards written English.
You can find it in many places online. Goodreads is one: