Technical writing benefits from Practical Style (the reader's time is important, the reader wants to get a job done, easy parsing of sentences is important), so I'd recommend one of the two best books about writing in English:

Joseph M. Williams, Style: Ten Lessons in Clarity and Grace.

An old edition is fine. There are many editions with slightly differing titles (Toward Clarity and Grace, The Basics of Clarity and Grace), all of them are fine. Get the cheapest or fastest to deliver or whatever. Don't think about which one to get.

The other great book about writing is Thomas&Turner's Clear and Simple as the Truth. It teaches Classical Style, which is less fitting to technical documentation, as the authors discuss themselves.