This reads like a novel, informative, and should be a novel (!) series. After reading this book I went out and brought the second book Nero's Killing Machine and his new novel The Inquest, I also have his third book on order.
His writing sytle is in a story format, akin to a novel. It is not dry but easy going, it took me two to three days to read it, fast for a non-fiction historical book.
Some problems that die-hards history buffs will not like is the Anachronistic use of modern equivalences. But the book, I don't think, is aimed at die-hards but for people who just want a basic understanding of Roman Legions.
For further, deeper reading, D-C is not for you, read the classics of that period or modern classics that focus specifically on details and understanding.
Like Theodore Ayrault Dodge, Edward Gibbon, or Polybius, Ammianus Marcellinus.
He is worth the time and money for anyone wanting an overview look at Roman History.