Recently I read “Growing Rails Applications in Practice” book, which I'd highly recommend to all Rails developers who are building or maintaining medium to large Ruby on Rails apps. The book is very practical — all the techniques from this book can be applied immediately. The book is also very short (just 88 pages), so it won't take much time to read.
The book helps to “unlearn bad Rails habits” by explaining how to get rid of fat controllers and models and still use native Rails idioms and patterns with smart namespacing and inheritance techniques. It provides an example of how to build user interactions without a database using plain models without database interactions, and how to refactor controllers with too many responsibilities. It also gives advice on taming stylesheets using BEM. The last part of the book covers unit and integration testing.