The subheadline reads "The definitive books on the battles of the 20th century," which ain't really true. Hanson recommends "The Price of Glory" by Alistair Horne, "With the Old Breed" by E.B. Sledge, "The Face of Battle" by John Keegan, "Stalingrad" by Antony Beevor, and "The Fall of Fortresses" by Elmer Bendiner.
Hanson is the man, but I'm not sure he's the guy to ask about books on 20th century battles. For example, only one-third of Keegan's book is even in the twentieth century. The Sledge and Bendiner books are memoirs, and while there is nothing wrong with that, it means that some other important books were left off. So here are a few supplements, mainly American, to the list:
1. "D-Day" by Stephen Ambrose--as good a book as you will find on one of America's largest battles.
2. "A Time for Trumpets: The Untold Story of the Battle of the Bulge" by Charles MacDonald--as good a book as you will find on America's largest battle.
3. "Hell in a Very Small Place: The Siege of Dien Bien Phu" by Bernard Fall--the definitive account of a French defeat.
4. "We Were Soldiers Once...And Young" by Harold Moore and Joseph Galloway--they made a movie out of the first half of this excellent recounting of the Battle for the Ia Drang Valley.
5. "Black Hawk Down" by Mark Bowden--they made a movie out of this one, too. Lots of military guys have nitpicky problems with Bowden, but overall it is excellent.
There are lots more--Barbara Tuchman on WWI, Richard Frank and George Feifer on the Pacific War, James Willbanks on Vietnam--but that's plenty for now.