As to be expected from such a successful fiction author, Waterloo is an easy-to-read book whose narrative helps make sense of a very confusing battlefield. Cornwell does a great job of not only setting the strategic and operational stage, helping us understand Wellington, Blucher, and Napoleon, but also weaving in personal anecdotes of the soldiers who fought and died on the battlefield.
Waterloo, as can be imagined in a battle its size, was confusing. Many accounts contradict each other or, at least, can't agree on the timing and sequence of events. Cornwell addresses many of the key disconnects and lays out what he believed happened all while telling a story. When it's done you can begin to feel how the battle flowed, not in neat phases but as a series of often overlapping events played out over the battlefield. In this the storyteller comes to the fore because he's able to weave together a variety of multiple events, perspectives, and people in such a way that you see the whole and not the part. It's not just a story of the British or Prussian or French armies, or the common soldier, or the three great leaders. In the end it's a story about how that all came together. In this Cornwell was able to rely on the original work of historians - Cornwell's strength in this book is not original research, there's nothing new historically, but what makes his book worth reading is the way he painlessly tells the tale in a very understandable manner.
If I had one complaint, and it's a relatively small one, it's that he switches between the past and present tenses in his writing, sometimes within the same paragraph. I think he periodically switched to the present tense to try to increase the immediacy of what he was describing, but, for me, it tended to interrupt the moment; rather than staying caught up in the story, it reminded me I was reading a book.
If you're familiar with some of the post-war finger-pointing among the allies you'll understand, after reading the book, Cornwell's conclusion that, "The battle of Waterloo was an allied victory. That was how it was planned and that was how it turned out. Wellington would never have made his stand if he thought for one moment that the Prussians would not let him down. Blucher would never have marched if he thought Wellington would cut and run."
This is one of those books that I still recommend even if you're familiar with Waterloo. Cornwell's ability to tell a story really pulls together the historical elements in a way that allows understanding.