Taking its title from a quote by German Field Marshal Erwin Rommel - "Believe me, Lang, the first twenty-four hours of the invasion will be decisive...the fate of Germany depends on its outcome...for the Allies, as well as Germany, it will be the longest day." - Ryan's book deals with the Normandy landings. Based on hundreds of interviews with Allied, German, and French participants, for many years The Longest Day was the best of the many books written about what Stephen E. Ambrose once called the climactic battle of World War II.
Ryan divides his book into three parts: The Wait, which tells how the Allies and the Germans prepared for the invasion; The Night, which describes the confusion and complications of the nighttime airborne assault; and The Day, which, of course, takes readers onto the five invasion beaches, where so many acts of bravery and so many horrific scenes took place. Ryan skillfully lets the accounts of the veterans from both warring sides blend into his crisp and clear prose, grabbing the reader's attention and never letting go.
The Longest Day was followed by many other works on the topic of D-Day by such authors as David Howarth, John Keegan, Paul Carell ("the German Cornelius Ryan"), Max Hastings, and, of course, the late Stephen Ambrose. It was also abridged in several issues of Readers' Digest (which lent its support in research and interviewing participants), and was adapted into a major motion picture by Darryl F. Zanuck and 20th Century Fox in 1962.
What really struck me is that Mr. Ryan wrote it so soon after the war (first published in 1959) and yet there isn't a note of triumphalism in the whole thing or a note of bragging as victors are sometimes wont to do. He is wonderfully objective and whatever biases he may have had, he subordinates to the story. He also never engages in what each side should have done as many historians do.
While this was written and published prior to the revelation of Ultra and the Enigma codes, it fails to diminish the book's power and readability.
If you want a good primer on D-Day what it felt like, then this is the best book to start.