There are several resources for learning about technical writing. Here's a collection of articles/books/other materials I've found useful.
- I'd Rather Be Writing by Tom Johnson
- FFeathers by Sarah Maddox
- Cherryleaf
Use bookshop.org to support your local bookstore.
- Technical Writing Process: The Simple, Five-Step Guide That Anyone Can Use by Kieran Morgan
- The Elements of Technical Writing by Thomas Pearsall
- Technical Writer's Handbook by Matt Young
- Technical writing classes from Google
- Season of Docs - bringing together Open Source communities and technical writers.