John H. Gill's three-volume work Thunder on the Danube is an outstanding study of the 1809 war with Austria. Not including the very extensive notes and references, it comes to about 1,000 pages over the three volumes, and I'd be lying if I said it was an easy read; it's very dense and scholarly in its style, with essentially a day-by-day breakdown of everything that happened in the war, from the very top (Napoleon and Charles's commands) all the way to the bottom (the experiences of individual soldiers). It's extraordinarily detailed and thoroughly researched, with dozens and dozens of detailed maps for every engagement and highly accurate tables of army compositions. The books present the history from both sides equally, though in many cases there is more accurate information available from the French/"Allied" perspective than the Austrian one. Because Gill speaks French and German he extensively uses primary sources, which for me is preferable.

I don't think a military history exists on a single campaign or war in the Napoleonic Era that is more thorough and detailed than this.