Stephen Ambrose writes some of the best historical non-fiction I have read (and I have read quite a lot.) His work regarding the European theater of WW2 is stellar--richly detailed and extremely well-researched.
This particular book deals with D-Day--there is some lead-up, details about how the invasion was planned and prepared for, but it is mostly just the events of June 6, 1944. Ambrose goes through each group that participated--paratroopers, bombers, sailors, infantry, engineers, Rangers, commandos, etc--and takes them from the staging ground up on to the beaches of Normandy. He explains, with the aid of photos and maps, where each group was, what their objectives were, and how successfully they completed the planned objectives.
The Allied invasion of France was a success due to several factors. The first was the ability of the Allies to create and produce the sheer amount of materials needed to move and equip a force of this size and nature. The second factor was the amount of planning that went into the process; the military personnel participating were trained so thoroughly that even unexpected changes and challenges were met without much panic. A third factor was the ability of the Allied troops to make adjustments--many of the commanding officers were killed early in the landing, and lower-level officers, noncoms, and even enlisted men were able (due partly to their extensive training) to stand up and lead their fellow men to the objectives. Another reason for success was the Allies' access to information--between Allied spies, Air Force photography, and members of the French resistance, the Allies were able get a very good idea of what would be waiting for them, which allowed them to plan accordingly. A final factor was the German command's almost opposite situation--the soldiers were ill-prepared (many were either too young or too old, and had spent little time combat training, and instead spent most of their time building the Atlantic wall defenses), their information was poor (the Allies' use of misdirection led the majority of German forces to be in the north of France, preparing for an assault on Norway), their troops were often disinterested (many were prisoners from the Eastern front who were perfectly happy to surrender), they had trouble moving the supplies they had (bombing raids had disabled many of the roads and train tracks, making it difficult to get any supplies to the area), and the command structure was set up in a way that did not allow for initiative--everything had to be approved, sometimes directly by Hitler himself. By the time the Germans were aware of what was happening, it was already too late.
This book is extremely detailed, and it's clear that the author was very diligent about his research. He quotes hundreds of interviews and oral histories from men involved in all areas of the conflict, so much of the book is told in the words of the men who lived it. That adds a wonderful feeling of realism to the whole thing. It is amazing, funny, exciting, and sad by turns.
I highly recommend this book to anyone who enjoys American history. It is both well-written and jam-packed with information.