If you want to start building your new web application and you picked to completely separate Client side and Server side codes completely. Then it's a high chance that you need to build API regardless of what you picked for your frontend.
I've used Ruby on Rails in my software house since 2011 and personally think that it is more than enough for most of my customers.
Now that Ruby on Rails has this "--api" option where you only use Rails to build API. No HTML, CSS, JS rendered from backend (except for emails). I only use this "--api" option for newer projects and moved my frontend to use NuxtJS, NextJS or AngularJS.
My life is now much simpler because I can now separate task between frontend and backend. My team work more effectively, and even better unit test.
This book is to show you how to write API on Rails. It's purely based on code and building prototype application.
This book will not cover Unit Test, I think it's better to start with the fun part and continue with the boring part in the next book.