I got this after reading Vanity Fair, and reflecting that it's a while since I last went down to the battle site, which is only half an hour's drive from us. It is a very short but very detailed account of the June 1815 campaign which sealed Napoleon's fate. The carnage was brutal and vicious; the battle of Waterloo took place over a very small area, four kilometres by two, with the focus of the fighting being two building complexes which Wellington needed to hold long enough for the Prussians under Blücher to arrive from the east; he held one and had to abandon the other when the garrison ran out of ammunition, but it had held for long enough for the Prussians to arrive.
Roberts is excellent on the details (and there are two very good maps) but very annoying in his description of the context. The British element of the allied forces get noticeably more praise for gallantry, bravery and intelligence (when basically the crucial move in the battle had happened a couple of days earlier, when Blücher's deputy decided to retreat north rather than east from their defeat at Ligny, and so were available to save Wellington at Waterloo). One senses that Roberts is trying to be neutral and objective, but that his heart is not in it. I did, however, appreciate his debunking of Victor Hugo's description of the battle in Les Miserables.
The other problem with the book is that it isn't made terribly clear why all this slaughter mattered. What if Wellington and Blücher had lost? Napoleon would eventually have been defeated by the Austrians and Russians, no doubt with the help of a revived British army of veterans from the war with America; or else (perhaps less likely) he might have settled for a restored Empire including Belgium but otherwise at peace with his neighbours. Europe in 1840 would surely not have looked very different if Waterloo had gone the other way (except, as noted, for Belgium). Waterloo put an end to Napoleon's career, but he had peaked in 1812 and it was always going to be downhill from there. In the end, I was actually left wondering if it was all necessary.