I absolutely adored The Subversive Copy Editor both as an editor's resource and a guideline to interpersonal relationships.
I've also enjoyed The Careful Writer by Theodore Bernstein, anything by Bill Walsh, and On Writing Well by William Zinsser.
I think Reedsy is a great place to find indie authors who are looking for editors. The site also looks for experienced editors so you'll probably fit right in!
The Editorial Freelancers Association has some pretty good resources/guidance on getting started. I'm also gonna recommend The Subversive Copyeditor, because it's honestly the best book I've come across in my career.