Hope you’re all doing alright. I wanted to share with you a book that I'm almost done reading: The Object-Oriented Thought Process by Matt Weisfeld.
After studying the Actor-Oriented design course I had questions specially related to abstract messaging between actors, building actors through composition and terminology in general on how the framework works. I thought that I had good OOP knowledge because I took NI’s course, but it wasn’t enough to really comprehend what lies behind the scenes. There are a lot of books that jump directly to OOP using a specific programming language but they do not focus on the main concepts. Matt describes the concepts very well using basic UML diagram examples, pictures and pseudo code.
Highly recommended if you want to review OOP concepts or you're starting your OOP journey.