As the title suggests, Exim is a Mail Transfer Agent (MTA). In other words it is a program which is responsible for moving email from one place to another.
As this book is written by Exim's author you would expect it to be both authoritative and accurate and it is both of those - there were no glaring errors or omissions in the text. In addition the book is well written and perfectly readable.
The book starts with a basic overview of how email works and moves from system to system across the Internet and then proceeds into an overview of Exim complete with a simple configuration that will suffice for many small sites. The next few chapters provide more detail on the operation of Exim and how to extend the configuration to cover more complex scenarios.
After that the majority of the book forms a detailed reference work covering both Exim and its configuration file and describing all the things which it can be made to do while handling email.
I have been running Exim for a number of years, both on my home systems and as the basis of a corporate mail system spanning three continents. With that amount of previous experience, much of what this book had to tell me was not new to me although it does serve as a useful reference. That said I did learn a number of things that I had not previously been aware of and got several ideas for improvements that could be made to the configuration of the systems I manage.
I have no hesitation in recommending this book to anybody with a need to manage an Exim based mail system of any size.