I'll start with the book's biggest flaw: Paul Dawson clearly has an axe to grind about the battle, and about the French Guard participation. While a little distracting at times, it doesn't detract from the overall value of the book. Using an impressive array of first-person accounts (including many that are privately held) and the surviving French after-battle loss reports, he goes through literally every battalion, squadron, and regiment in the battle and details the part it played.
That is a stunning and eye-opening feat that reveals a lot of detail that has been previously mis represented. The battles for Hugomont take on a whole new shape with a very different focus, particularly in the early phases where the French attempted to outflank the wood and assault the chateau from the west, not resorting to assaults from the woods until quite a bit later.
Likewise, the battle against the Nassauers and Prussians on the eastern flank takes on far more nuance, with the Young Guard initially being committed against Samohain to hold Lobau's northern flank when Ist Corps started failing to hold back the Nassauers around Papelot. The brigade committed here continued to fight on Lobau's left for the remainder of the battle, holding the critical juncture in the French lines.
Even the much studied D'Erlon's Charge gains a lot more clarity when the battalion-by battalion losses are compared with personal accounts.
All-in-all a great book flawed by being a little tedious and the aforementioned axe being ground.