The most recent version for Rails 8.1 is the last update to Sustainable Web Development with Ruby on Rails. As of now, this book is out of print, and I don't expect to be updating it.
The practical reason is that I do not use Ruby on Rails and have not for the past few years. It's important to me that technical books like mine contain practical information, based on real-world use. That's what Sustainable Rails has been. But, as my distance from Rails increases, that will be less and less the case.
There are two reasons I'm not using Rails. The first is a pragmatic one: I haven't had the need to use it for a job or project (for personal projects, I'm working on and with BrutRB, my own web framework). The second is more ideological. The creator of Rails has repeatedly shown public support for moral views that I not only disagree with, but find offensive and intolerable.
I'm lucky enough that I don't depend on Rails for my livelihood, so I'm taking this opportunity to simply stop tolerating him. A few web searches can provide you with the details if you are interested, but if this doesn't matter to you, that's fine - this decision is personal and I'm not trying to convince anyone of anything.
I'd also remind you that this book is under copyright, so please do not distribute your copy. You are encouraged to share what you've learned with others, but the book itself is out of print, not in the public domain.
Lastly, I want to thank everyone that bought or recommended this book. I wrote it as a way to get my experience with Rails out of my head and into a format others would find useful. I hope you've found it useful!