These are some of the best ruby books in my opinion, that can help you go to the next level. They are easy to read and every ruby developer should read them at least once.
1. The Well-Grounded Rubyist
If you are you new to ruby, you will need to know how ruby works, and how it's structured. This book will give you a solid understanding about ruby and its unique design.
Practical Object-Oriented Design in Ruby: An Agile Primer (Addison-Wesley Professional Ruby)
This book will get you to the next level in Object Oriented Programming in general, no matter the language. But it's written with ruby examples which you can easily use in your day-to-day life as a developer. It includes OOP design principles in compact, well structured ways.
Refactoring: Ruby Edition: Ruby Edition (Addison-Wesley Professional Ruby)
Clean code matters, a good developer knows how to refactor code correctly. This book is a rewrite of the original book Refactoring(Java) but with ruby examples.
4. Design Patterns in Ruby (Addison-Wesley Professional Ruby)
Like OOP, design patterns in software engineering will help you with any programming language. This is written in ruby of course and it will help you identify and use common design patterns in your code.
These are the books I've chosen, that can get you to the next level in ruby. There are many others, but these are the ones that are easy to read and can transform you into a better software engineering in general.
You will need to read them more than once or visit specific chapters when you need to remember something in particular.
Don't forget that a good ruby developer knows how to write good tests. Do you know any good TDD books? Personally I've used the official RSpec guides. Comment below if a book has helped you write better ruby tests.