I wrote a book titled "Ruby on Rails Step Up" for elementary and intermediate Rails users.
If you subscribe to Kindle Unlimited, you can read them at no additional cost.

https://www.amazon.com/dp/B0BST6TM31

Please read it if you interested :)

About this book

This book introduces and explains a wide range of techniques and knowledge in Rails team development, such as refactoring methods, gems, service classes, RSpec, and frontend.
Conceptual explanations are the main focus, and sample code is somewhat little. The concrete implementation methods are not intended to be solved in this book alone, but rather to be Googled in conjunction with the book.

Intended Reader

This book is intended for elementary and intermediate Rails users, and assumes that the reader has some experience with Rails. So, it is not recommended for programming beginners or those just starting out with Rails.

• People who are not familiar with systems other than those built by their own company, and people who do not understand other parts of the system for which they are responsible for.
• People who have never written an RSpec before, or who have written one but feel they do not understand it in depth.

For these people, this book is recommended.

Table of Contents

1. Preface
2. Controller
3. Model
4. View, Helper, and Decorator
5. Service Classes and Directories under app
6. Routing (How to write routes.rb)
7. DB
8. Other Techniques 1. Frequently required features in Rails
9. Other Techniques 2. Other tools in Rails
10. Other Techniques 3. Team development
11. RSpec - 1. Types of specs and how to use them
12. RSpec - 2. Commonly used gems and their usage
13. RSpec - 3. FAQs about testing (benefits of testing and attitudes toward testing)
14. Frontend in Rails - 1. Asset Pipeline and Webpacker
15. Frontend in Rails - 2. Often used tools and terms
16. Overview of the skills needed in Rails

Main Points

• 05. Service Classes and Directories under app
  • Recommended implementation policies are included.
• 11~13 RSpec
• 14~15 Frontend in Rails
  • It mentions plain Javascript, jQuery, Vue.js and React.

Other

The content is based on Rails 6.1.6 and Ruby 3.0.3.
This book refers to the systems of Tokyo-based companies, some of which are well-known.

About the author

I am currently a freelance SE living in Tokyo with about 8 years of Rails experience.
Articles I have written in the past:

Refactoring guide for Rails beginners

Thanks!