Elgar's Variations for Orchestra, commonly known as the 'Enigma' Variations, marked an epoch both in his career, and in the renaissance of English music at the turn of the century. The first extended study of the work, this Cambridge Music Handbook contains historical information concerning the conception and writing of the work, an extended musical discussion requiring only a little technical knowledge of music, and a fascinating survey of the "solutions" to the mystery implied by the title of Elgar's most famous work.
It is a fairly comprehensive book about Elgar's Enigma Variations, though incredibly biased about the enigma meaning, or lack thereof. It also doesn't go in depth about the many other interpretations of the enigma that have been vocalized, you can take that as a good thing or as something the book is lacking. With that being said, it is still a comprehensive guide and a must-read for anyone interested in the history of this great work.