I have read several of the books in the Cambridge Music Handbooks Series. The purpose of the books in this series is to aid the listener in understanding the theoretical/analytical aspects of the music (such as form, recurring thematic material, growth process, etc.) as well as the historical background of the piece, including critical reaction to the premiere of the work.
The book is divided logically into four chapters. The first chapter deals with Britten's pacifist beliefs and how they led to his encounter with the poetry of Wilfred Owen, a soldier who had been killed in World War I. Owen turned out a small body of poetry during the last two years of the war, nine of which Britten chose to use in his Requiem, along with the text for the Latin Mass of the Dead.
The second chapter gives the historical background of the piece: commission, composition, and the premiere performance.
The third chapter is subtitled "The musical language: idiom and structure." This chapter details the musical content of the piece and how it conforms or deviates from the traditional Requiem formula set up by Mozart and Verdi. This chapter is the "meat" of the book as far as this critic is concerned. In understanding the musical content, one is better equipped to listen with an ear of understanding instead of ignorance.
The final chapter reveals the critical reception of the piece, which was extremely positive. Most critics immediately hailed this work as Britten's masterpiece. As the author states, "it is difficult to call to mind any other major twentieth-century work which met with such instantaneous and unanimously high praise from almost all sectors of the media."
In conclusion, I can highly recommend this book without hesitation if you are at all interested in the music of the twentieth century. Britten was clearly a brilliant composer; the War Requiem is, arguably, his masterpiece; and this book is a fine tribute to a wonderful piece of music.