In a way, I learned about Napoleon as a commander backwards. My interest in all things Napoleonic initially grew out of my reading of Regency romances, where the occasional hero is a veteran who served under Wellington. Nowadays it's rare, but in the 80's and 90's one sometimes ran across a Regency set in Brussels in the run-up to Waterloo, at the Congress of Vienna, or in Spain or Portugal during the Peninsular Campaign. Those were always my favorites, since I had a lurking interest in military history (possibly because one of my brothers started West Point the year I started kindergarten), and I liked the higher stakes than your typical Regency set against London high society or the pastoral English countryside.
Then I stumbled across Bernard Cornwell's Sharpe series. I have to confess I started with the TV movies, after watching The Fellowship of the Ring and wanting to know what else the Boromir actor was in. But I read the books, too, and started researching the history behind them. Which meant mostly the Peninsular War, and mostly Wellington. Like the Great Duke, I first encountered Napoleon at Waterloo. As my interest in the era and the people broadened and I started to want to write about it myself, I learned more about Napoleon, but I still know Waterloo better than anything else from his career.
I picked up Three Napoleonic Battles (Harold T. Parker, 1944) as part of my ongoing effort to get a broader perspective on Napoleon's career. Parker analyzes the battles of Friedland (1807), Aspern-Essling (1809), and, yes, Waterloo (1815). In the first, Napoleon was at the top of his game and facing a Russian commander who made serious blunders, in the second he made blunders of his own against a solid opponent and was checked, but through his and his marshals' skill and his opponent's overcaution avoided a crushing defeat, and, well, we know what happened at Waterloo. (At least I hope we do. If you're reading this and thinking, "I vaguely recall Napoleon from high school history class, but isn't Wellington a boot and Waterloo an ABBA song?" please talk to me. I have books to recommend. You'll love them, I promise. History is exciting, and 1789-1815 especially so.) For the most part, Parker's conclusions are pretty straightforward--Napoleon got older. His health and energy declined, enough that he lost a mental step, too, though he was obviously still beyond unusually intelligent. His natural optimism was perhaps less tempered by realism--e.g. his stubborn refusal to believe the evidence that the entire Prussian army was indeed marching to unite with the Anglo-Dutch forces at Waterloo. Plus, his opponents got better, both in the sense of learning his game plan and that over decades of war, the cream of the other powers' generals eventually rose to the top.
For all that, it's a worthwhile and well-researched read, dense with detail. I expect I'll be turning back to the Friedland and Aspern-Essling sections as I develop my alternative history, and I never get tired of learning more about Waterloo. Wellington once commented (I'm paraphrasing wildly here) that you might as well write the history of a ball as of a battle--everyone who was there gives a different account, and it's impossible to pin down what really happened. To me, that's precisely what makes Waterloo especially so endlessly fascinating. Every time you look at it from another angle, you get another facet of the glorious horrible chaotic epic of it all.