If you'd want to try again, I can recommend "Elixir in Action" [1]. It has a great introduction to the basics of OTP later on in the book. I understood it much better after reading that book that any other materials.

While the book is about Elixir (which is a BEAM language) the underlying OTP principles are the same. The book covers "basic" OTP before moving on to some abstractions that Elixir provides.

[1] https://www.manning.com/books/elixir-in-action-second-editio...