Reddit mentions of Practical Object-Oriented Design: An Agile Primer Using Ruby (2nd Edition)

Practical Object-Oriented Design by Sandi Metz and Refactoring: Ruby Edition by Jay Fields, Shane Harvie, and Martin Fowler with Kent Beck were both invaluable for me. They’re more Ruby focused than Rails, but these will help with Rails and so much more.

I apply what I learned in these books in Python, PHP, Ruby, and any other object oriented language I happen to be using.

I'm getting good mileage out of David Copeland's Build Awesome Command-Line Applications in Ruby 2 (2013). For Ruby-specific best practices (I'm coming from PHP), Sandi Metz' Practical Object-Oriented Design (2019) and Russ Olsen's Eloquent Ruby (2011) are opening my eyes to how it's done here. In particular, Metz' focus on the role of messages in OO design has changed my approach to planning and testing - just in time for a critical project.

This book helped me, well, the first edition did:

https://www.amazon.com/Practical-Object-Oriented-Design-Agile-Primer/dp/0134456475/ref=sr_1_9?ie=UTF8&qid=1537986763&sr=8-9&keywords=ruby+programming

It's fairly short, and Ruby is easy enough to understand what's going on here. This really made everything click for me.

The best book on OOP design I've found is this one. I highly recommend it. It's in Ruby, not Python, but this should not matter at all. OOP is not specific to Python (if anything, Python programmers care a lot less about OOP than, say, Java programmers). You can try to re-implement the examples in Python and now you have extra practice.