I recently read Modern Technical Writing by Andrew Etter and I highly recommend it. It is a very brief but cogent intro to the field. The book also has a slight slant toward developer docs, so that may work well for you.
The “Modern” in the title is justified. I found the book refreshing, partly because he drops a lot of the old STC-style mentality (read the intro–he’s too polite to point the finger, but we veterans are not fooled) in favor of an approach that emphasizes hands-on learning, using simple tools, and getting the devs to contribute.
I believe this is only available as an eBook on Amazon.
Another excellent intro is Technical Writing 101 by Sara O’Keefe and Alan Pringle. Also an eBook on Amazon.