A quick read which I would recommend to every mid rails developer and higher. It's common for developers to outgrow their tools. Rather than learn new skills, people start to blame the tools. Your heard those people: "Rails does not scale", "ActiveRecord is antipattern" etc. The books show few patterns which allow to build big Rails application with only small changes to standard 'Rails way'.
The book recommends active_type gem. I would recommend other gem active_interaction instead. active_interaction gem is more superior in my opinion and I've successfully used it in multiple projects over the last 2 years.
As for the CSS structuring, BEM is definitely a good choice to get control over your styles. Only issue with it, if not enforced it loosed its value, so every developer should be very diciplined to follow BEM.
There is only 1 option which is better than BEM, it's css modules. I only used it with ember.js, not Rails though. It automates steps that you have to do with BEM. However css modules require deep integration into the tools you use, which makes it harder to bring into the project.