Rory Muir, Wellington: The Path to Victory 1769-1814 and Wellington: Waterloo and the Fortunes of Peace (Yale University Press, 2013 and 2015).
These two books are, in my humble opinion, a phenomenal achievement. Based on almost two decades of research, they offer the most comprehensive overview of Wellington's life that has ever been attempted. People will no doubt make comparisons with Elizabeth Longford's biography (also originally published in two volumes, though since reprinted by Abacus as one volume). Longford's work is good, was very good for its time, but Muir's is much more detailed, and draws upon a greater breadth of material.
I particularly like the fact that Muir is not seduced by the 'great man' discussions which so often clutter biographical work on historical figures. There are some eye-opening revelations (I'll let readers enjoy discovering those for themselves), but also a great deal of balanced analysis. Wellington is, rightly, not placed upon a pedestal, but analysed, critised and praised depending on the situation.
The books are very reasonably priced given their size (book one is c. 900 pages, book 2 not far off). Muir's work has been highly praised, winning the Society for Army Historical Research's Templer Medal.
Particularly useful from a researcher's perspective is the fact that a wealth of additional side notes, references, and other useful material (amounting to many hundreds of pages in their own right) have been made freely available online: Wellington: The Path to Victory 1769-1814; Waterloo and the Fortunes of Peace 1814–1852 (lifeofwellington.co.uk)