This is, practically, an encyclopedic sourcebook, which provides a short introduction to the authors of the detective fiction. Like Gian Piero Brunetta's "History of Italian Cinema" it is practically useless without knowing the background works but it is a very good exhibition of the periods and the development of (largely) English-language detective fiction (whodunnits, but also other subgenres). As is usual in such a wide canvasses, it is sometimes wanting in its accuracy.
For instance, for the creator of Maigret, Georges Simenon -- the only foreign author with a detailed exposition in the book -- his Nazi collaboration is mentioned in a few oblique and incorrect words. In fact, during the Nazi occupation of France the literature of the kind "How to uncover a Jew" (Communists, Resistance members, etc.) was flourishing and Simenon was one of the most prolific authors. These were not some anti-Semitic rants for which more famous collaborators (Brasillach or Mac Orlan) were known, but detailed instructions of what to ask children (how you spent your last Christmas, can you sing carols for me, etc.), or how to recognize forged ID papers.
I thank Edwards profusely for mentioning R. van Gulik and putting him in the context of the Dutch detective tradition. Yet, he fails to mention that van Gulik's fame was created by his magisterial non-fiction treatise "Sexual life in Ancient China", which stirred quite a shock in the puritan 1950s and immediately put its author on the literary map.
Obviously, in a treatise of such magnitude as Edwards' errors in fact and judgment are inevitable but they cast a shadow over accuracy of other facts, with which I am not familiar.