To be honest I have mixed opinions on this book because on the one hand it brings something new to the exhaustive study of Napoleon's great defeat in a Belgian field nearly two hundred years ago. On the other hand, this book has some very strong biases which make me afraid Hamilton-Williams may fall into the same traps that have mired much of Napoleonic Wars scholarship for the past two centuries. Overall I found the book very educational and exhaustively researched, but much was overshadowed by Hamilton-Williams's vitriol for the Bourbon dynasty and their supporters.
To understand the problems with traditional Waterloo scholarship, you have to go back to Captain William Siborne and his models of the battle. According to Hamilton-Williams, Siborne created these models as an economic enterprise and relied upon the statements (and funding) of veteran officers of that campaign. Because Siborne only consulted British sources, and was obligated to exaggerate the roles of his financial backers, his models ended up with a very distorted view of the battlefield which largely disregarded the roles of the Dutch, Belgian, and German soldiers which made up some two-thirds of Wellington's command. The result, by the 1880's, was a myth of British military invincibility and the inability of Wellington to make a mistake. Even on the French side, Napoleon is claimed to be incapable of making a mistake and the blame for Waterloo is heaped upon his marshals, Ney and Grouchy. Hamilton-Williams is not afraid to give blame when it is due, even to the great commanders, and provides what I found to be a largely even-handed account of the Hundred Days and the campaign that culminated in Napoleon's rout at Waterloo.
In Hamilton-Williams's favor is the exhaustive research and extensive footnotes contained within his text. Furthermore Hamilton-Williams draws upon British, French, Dutch, and German sources to provide a comprehensive account that transcends national agendas. (Although I have read that there may be some controversy regarding Hamilton-Williams's sources. I was unable to find anything concrete regarding these accusations, however if more information comes to light I am willing to reanalyze.) With a wide range of sources and detailing specifically how Siborne's account perpetuated unfounded assumptions were far from the truth. The result is a rather complete picture of the 1815 campaign that challenges long-standing assumptions.
Despite his exhaustive research, which may have its own controversies, Hamilton-Williams seems to be fixated on two subjects which tends to cause detriment to the rest of his work. The first issue is Hamilton Williams's adamant defense of the Dutch and Belgian soldiers under Wellington's command who were accused in the Siborne version of events of extreme cowardice and were the cause of all the pitfalls of Wellington's efforts. There are certainly extensive sources, at least according to Hamilton-Williams's footnotes, which certainly state that the Dutch and Belgian soldiers acquitted themselves just as well as Wellington's vaunted redcoats. The trouble is that Hamilton-Williams focuses almost exclusively on the Dutch and Belgian troops and provides relatively little information about the British and German troops that formed up Wellington's army. Although the British soldiers present were probably well-represented in other, Siborne-friendly sources, the relative lack of information leaves large gaps in Hamilton-Williams's account. I feel that in trying to defend the Dutch and Belgians Hamilton-Williams lost a certain degree of perspective.
The other major issue was Hamilton-Williams's vitriol for the Bourbon dynasty of France and their supporters and he makes absolutely no attempt to hide it in his book. In fact in the front-matter of this text he specifically picks out the Bourbons as the most despicable characters in the events of 1814 and 1815 and never hesitates to get a shot in at their expense. While there were certainly issues with the Bourbon regime and Napoleon was definitely a force for change in France, I feel like Hamilton-Williams let his own prejudices gain the better of him and prevented him from providing an impartial appraisal. Certainly bias is a part of any writing, but many historians today aspire to avoid the level of bias found in Hamilton-Williams's book.
I think that this book is very informative for students of the Napoleonic wars, but it falls short in some respects. Despite its exhaustive research the book's bias is incredibly strong and it goes out of its way to pursue that bias in some cases. I was hoping to find more material on the controversy regarding this book but since it's about twenty years old I'm afraid more information will likely not be forthcoming. Definitely look at other sources for more Waterloo research.