If you had to choose one to start would it be Sustainable Rails or Agile Web Development with Rails 8?

Hmm, good question. I think I'd probably start with Agile Web Development with Rails 8.

It's the book I learned originally Rails with more years ago than I care to remember (pre version 1!).

Sustainable Rails is really more about how build bigger projects over the longer term.

i.e. good practice for reducing the churn on model files when you have a larger teams or how to do things with a miniumum of external dependencies.

It's great stuff but you can probably pick that up later if you want to.

BTW I just saw this in browser tutorial from learning Rails[1]. I've not run fully through it but it looks really promising to me.

1. https://news.ycombinator.com/item?id=44574654