Erlang an OTP in Action has the deepest coverage of OTP of all the Erlang books I have read.
I recommend getting both it and Francesco and Simon's Erlang Programming to get a full appreciation of the language. Erlang Programming will get you mastery in writing code, while Erlang and OTP in Action will get you mastery in architecting it for the real world.
Yes, this is the main point with this book - the other existing books are mainly about Erlang itself, and do not cover the OTP framework in any depth. Our book is about using the OTP framework to write real-world systems, and assumes that you will read another book if you want to know more about the Erlang language.
However, chapter 2 of our book is an "in a nutshell" summary of Erlang as a programming language, so that you have a good reference at hand while you read the rest of the book.