The author, Robert Baden-Powell, was a British soldier and spy, and founder of the Boy Scouts. The author's love of adventure is evident in this book, which is a collection of 1- and 2-page essays.
After a short series of essays defending espionage as being both useful and honorable, the remainder of the book explains spycraft of the time and particular incidents in which Baden-Powell directly participated. The stories are interesting, and would certainly appeal to many young readers. Occasional use is made of the author's own illustrations, including sketches of fortifications modified for secrecy to resemble much more benign subjects (insects, stained glass windows, etc.) which were spirited back to headquarters.
On a historical note, this book is reminiscent of other adventure books by European authors in roughly the same period (Jules Verne, Edgar Rice Burroughs, Sir Arthur Conan Doyle).
Titles published by Dover tend to be inexpensive, and this book is no exception. Even at full retail, this book is a steal.