Start with An Army at Dawn. Rick Atkinson has written two trilogies and three earlier reported books, and the trilogies are strictly ordered, so the only real question is which trilogy to enter and where. The answer is the first volume of the Liberation Trilogy, because North Africa in 1942 and 1943 is a self-contained campaign with a clear beginning and end. If Atkinson's method works for you across 700 pages of it, the remaining 5,000 pages will too.
The obvious alternative is to jump straight to The Guns at Last Light, the D-Day volume, because that is the campaign most readers actually want. It is the worst decision available here. The trilogy's cumulative argument is about institutional learning — the army that crosses the Rhine is not the army that landed in Morocco — and the late commanders' decisions are only legible as products of the earlier ones. Read the third book alone and you get a very good narrative with its whole point removed.
Set expectations on size before you start. Every volume of both trilogies runs between 700 and 900 pages, with dense maps and roughly 150 pages of notes at the back. Thirty pages a sitting is sustainable; sixty will burn you out.
Start with the Liberation Trilogy
An Army at Dawn covers Operation Torch and the North African campaign. The first eighty pages are the slowest, because Atkinson introduces the American command structure, the British command structure and the French colonial position at once. After the landings it becomes narrative and moves considerably faster.
His method is visible immediately: a whole theatre narrated at the level of the individual soldier, with a general's decision and a private's letter in the same paragraph. Logistics gets as much attention as battle — fuel, ports, shipping and mud — which is a deliberate corrective to the great-captains tradition.
Finish the Liberation Trilogy
The Day of Battle, then The Guns at Last Light. This is the largest single commitment on the path, about 1,710 pages, and it should not be rushed.
The Day of Battle is the hardest of the three to read, and honestly so: Sicily and Italy were a grinding, often futile campaign and the book does not pretend otherwise, so the stretch around Anzio and Cassino can feel airless. It is also the volume that most complicates the good-war story. The Guns at Last Light is longer but reads faster, because the campaign has forward motion, and its scale forces a different technique — more theatre-level, less single-division.
The Revolution Trilogy
The British Are Coming, then Fate of the Day. The same method transplanted to 1775: disease, weather, shipping and distance treated as primary causes rather than context, and the British side taken seriously as a story with its own commanders and domestic politics.
The British Are Coming is the more propulsive of the two, because 1775 to 1777 has a natural narrative shape. Fate of the Day covers the war's longest and least decisive years and is correspondingly harder work.
Be aware before you start it: this trilogy is unfinished. Two volumes are published and the third is still to come, so this stage leaves you mid-project. That is a reason to read them now if you like the method and a reason to wait if unfinished series annoy you, but it should not be a surprise.
The earlier books
The long gray line, Crusade, In the Company of Soldiers. Much easier reading than the trilogies — reported books with smaller casts and no campaign maps to consult.
The long gray line follows the West Point class of 1966 through Vietnam and after; it is his first book and arguably still his most moving. Crusade reports the Persian Gulf War from close range. In the Company of Soldiers is two months embedded with the 101st Airborne in 2003, written with no hindsight at all, and makes a good final read because it shows the reporter working without the historian's distance.
Where to stop
The Liberation Trilogy alone is a complete work and the strongest thing he has done. Stopping there is entirely reasonable. If you want more of the same eye applied to a different war, Erik Larson is the closest adjacent shelf.
Follow the full ordered path here: Rick Atkinson Reading Order: Where to Start.