I'm a history major, for the record. I've heard all manner of semi-scholarly critisism of these Osprey books so I'll sum it all up and rather than give the book a one star for what it isn't, I'll review it for what it is.
These books are very entertaining introductory histories of their various (military) topics. The illustrations demonstrate their intent quite clearly: to entertain as much as to inform. These books are intended for laymen who enjoy military history, possibly the re-enactment crowd, and definitely bright adolescents. As "Illustrated Histories" go, they're quite excellent and I'm collecting them for my kids.
*WARNING-THESE BOOKS ARE NOT APPROPRIATE AS SCHOLARLY RESEARCH MATERIALS*
There are no footnotes, endnotes, citations, and no bibliography. There is the occasional (usually relatively minor) inaccuracy which is not a big deal for kids and laymen but is a major issue for academics.
That said, these books could help you out as introductions to material. For example, I bought and read this one as a general introduction to Assyrian military practices. It gave me enough of a grasp of the material to focus the thesis of my paper and know what kinds of things I should research. I did not use this book as a source!!
It is what it is, and as an entertaining introductory read for youths and laymen it is great. You ought not to expect a mass market paperback to live up to academic standards. If thats what you want, you should know that books meeting those standards are usually published by "(Insert name of Institution) Univeristy Press" or are primary sources (people who were there and wrote down what they saw).