If you have a technical background and you've coauthored a book, you already have what it takes to find full time work as a technical writer. There are really two ways into this field. You start with a background in English or professional writing and learn the technology, or you can start with a computer science degree or something similar and learn how to write. It sounds like you've done the latter.
The pay is good if you work for a company full time. Many technical writing jobs these days are remote. (All of the tech writers that work for Fastly and GitHub are remote, for example.) You could probably do freelance work, but I've never been able to make that work. Most companies don't outsource their documentation and I'm not entirely sure why, although I suspect it has something to do with them wanting to build long term relationships with the individual(s) documenting their products.
You will need to do some networking. You may want to consider attending Write the Docs (http://www.writethedocs.org/), a conference for technical writers. Depending on where you live, you might also be able to find an active STC group.
Or just start applying for jobs. :) Really good technical writers are hard to come by, so if you know technology and you can write, you'll probably have the job offers flooding your inbox. This recent posting at DataDog is a good example of something you will want to keep a lookout for:
https://www.datadoghq.com/careers/detail/?gh_jid=169167
Hit me up if you have questions.