Is this useful for non Rails users?
As the author, I’m biased, but I think so. While all of the application code examples are in Ruby and Active Record, there are many types of code or configuration throughout the book that aren’t.
For example, there’s SQL, shell scripts, SQL Query functions, PL/pgSQL procedures, and native database objects like constraints, views, or cursors that readers work with.
Don’t take it from me though. How about these responses from readers that work primarily with other technologies? 
Robert T. wrote:
Excited to see this hit the market. While Rails is in the title, don’t be put off if you work with something else; the Rails ecosystem provides a wealth of developer tooling and Ruby is an easily accessible language for studying, and the information can definitely be adapted to different domains.
Dave C. said:
"A book with this information would’ve allowed me to shortcut several years off of learning Postgres the hard way."
Haki B. wrote (after reading portions of the book):
“It’s amazing how much Django and rails resemble each other.”
My estimation is the book is about 75% PostgreSQL and about 25% Ruby on Rails.
My hope is that the book is useful even for programmers that work with other MVC full-stack web frameworks like Django (Python) or Laravel (PHP). Those frameworks will have ORMs with some overlap with Active Record, and when they connect to PostgreSQL, may have some of the same challenges around writing high performance queries.
Other topics like schema design, indexing, or maintenance, all have less to do with specific web frameworks or programming languages.
I also discussed with Drew Bragg on the podcast “Code and the Coding Coders who Code it Episode 27 - Andrew Atkinson” at the 33:50 mark:
https://www.listennotes.com/podcasts/code-and-the/episode-27-andrew-atkinson-0uJ_Yc82Npv/
While I originally considered positioning the book more broadly as something like: “PostgreSQL for Web Developers”, I didn’t think that would work out as well.
The main reason was because I wanted to have lots of concrete examples and exercises, inspired by my career experience from working with Ruby on Rails and PostgreSQL over the last decade.
To do that, the book uses more than 40 libraries from the broader ecosystem, as PostgreSQL extensions and Ruby gems. Most (not all) of the open source libraries that are included are there because I have firsthand experience using them in production. It felt more authentic to stick with what I knew best and have put into production and maintained myself. For libraries where that’s not the case, I try and call that out.
Hope that helps! Thanks for your interest!