Despite its gargantuan length and seeming density, this is not an "academic" treatise on guerrilla warfare. The book's approach is chronological: one chapter or two is devoted to a particular instance of guerrilla warfare, beginning in antiquity and ending with the Viet Nam revolutionary wars.* Many chapters seem to be little more than syntheses of a handful of secondary sources on that particular conflict. (For example, the chapters on the 19th-century Philippine insurrection against the US occupation read like a book report on Wolff's "Little Brown Brother," so heavily does the author rely upon that classic work). But taken as a "popular" history, Asprey's book is first-rate. His writing style is engaging, and his copious use of quotations from contemporary observers and participants helps bring these (sometimes relatively obscure) conflicts to vivid life.
I only have two real complaints. First, Asprey relies a little too heavily on the accounts of journalists and counterinsurgents, rather than the firsthand accounts of the very guerrillas whose history he claims to be writing. (There are obvious exceptions to this complaint, e.g. the writings of T.E. Lawrence and General Grivas figure prominently in the footnotes). I realize that village-bred guerrillas don't tend to publish as many memoirs as do former colonial generals, but such sources are available if one scours the archives closely enough. (Perhaps Asprey lacked the resources to do this, given the millenia-long historical scope of his narrative). Second, the book focuses mainly upon events rather than tactics. In other words, this is primarily a book about history rather than military science. Those interested in the nuts-and-bolts of guerrilla tactics and strategy would be better served by other books on the subject.
That aside, Asprey more than held my interest for 1500-odd pages, and I did learn a great deal even about wars with which I already had some familiarity. Highly recommended.
*Mine is the first edition, so I haven't read the updated material on Afghanistan and Central America that appeared in later editions. Nor am I aware if Asprey has revised any of the original material since the first edition. Note that in my edition, most of the second volume is devoted to the Viet Nam wars. I don't know if this material was shortened for the later editions.