"Wellington as Military Commander" by Michael Glover. Originally published in 1968, republished as part of Penguin's "Classic Military History" series.
I'm not terribly thrilled with it.
It's more of a career biography or a chronicle of campaigns than a critical analysis (the title implies analysis, but it fails to deliver).
Although it is, still after 42 years, considered to be one of the foremost works on the career of Sir Arthur Wellesley.