Bloomberg: If The American Civil War has a defect, it is the book’s less-than-chronological, sometimes jumbled narrative style.
Slate: Some of Keegan's puzzling judgments may reflect his very limited range of sources. [...] Some chapters cite no sources at all, and many rely heavily on James McPherson's modern narrative history, Battle Cry of Freedom.
London Times Online: Readers already expert in the subject will find much to debate in Keegan’s pages; those in search of an overview may well emerge more bewildered than enlightened.
Plain Dealer: ... it is obvious from the text and notes that Keegan has not really consulted the voluminous relevant secondary sources, much less done any original research... The book has a slapped-together feel...
Spectator [London]: Footnotes are so spasmodic that the criteria for citing sources are impossible to discern. Keegan has to be taken, for the most part, on trust. But his command of the war’s geography, his thorough understanding of military organisation and his deep humanity, all nourished by a lifetime’s immersion in military history, imbue his account with the authority that we have come to expect from him.
HNN: ...replete with a shocking number of seemingly inexplicable errors...major errors of fact which would embarrass an undergraduate.
Whoops - that last link is for a book Keegan published in 2003, containing one of his first forays into the ACW and containing howlers such as
The only check to the Union's steamroller advance into what had formed so much of the Old Northwest had been imposed in early April at a tiny place called Shiloh, far down the Tennessee River....Seek out the general reader of military books my friends - clearly, you can bluff him for the rest of your writing life.