It has been some time since I read that C.Ryan novel as well, but in all I have nothing but good memories of it. I believed the movie rendition was reasonably "true" to the story, but the book was better since it happened "in your own mind". Well worth the purchase price whatever you pay.
I think one of my favorite bits from The Longest Day, was Maj Pluskat trying to convince his superiors that the invasion was outside his pillbox. "Maj. Werner Pluskat: [on the phone again] You know those five thousand ships you say the Allies haven't got? Well, they've got them! Another good set of books is Paul Carrell's: "Invasion: They're Coming," "Hitler Moves East," "Scorched Earth," and "The Foxes of the Desert." Mind you Paul Carell (born Paul Karl Schmidt; 2 November 1911, Kelbra – June 1997) was an Obersturmbannführer (Lieutenant Colonel) in the civilian Allgemeine SS. He worked as the chief press spokesman for Nazi Germany's Foreign Minister Joachim von Ribbentrop. After World War II ended in 1945, he became a successful author. So some things are glossed over you might say, but still good reading.
I've read it 3 times (I re-read it every few years), and I have nothing but praise. Not only does it give you all the details, but it does so in an interesting way. I'll put it this way - you could probably read all 500+ pages of it in 2 nights. Its hard to put down once you start reading it. "A Bridge Too Far" and "The Last Battle" are also very good.
The WW2 books that left an impression on me and are a permanent part of my library: Shattered Sword - an outstanding book on the Battle of Midway and one that explains the basics of Carrier warfare in some detail - debinking many myths along the way. Eastern Front Books: Kursk - By Glantz and House - The best account I've read of the battle of Kursk. Stalingrad - Anthony Beevor - Likewise for the battle of Stalingrad 900 Days - a moving account of the seige of Leningrad Moscow 1941 - A city and its people at war (I lived in Moscow and related to the book a lot). All 3 books by Cornelius Ryan The struggle for the Middle sea (Vincent O'Hara) - detailed account of Naval warfare in the Meditterenean. HMS Ulysees (Alistair Maclean) - Read it as a kid. Left a great impression and started my love for the sea and the Navy.
This maybe is my favorite book, and I highly recommend it to anyone who has not read it, "The Bedford Boys" by Alex Kershaw. Kershaw does a great job presenting the compelling story of the men from the small town that paid the ultimate D-Day price. Give it a look you won't be sorry.
I can recommend Amazon.com: Hirschfeld (Cassell Military Paperbacks) (9780304354986): Wolfgang Hirschfeld: Books a great book about life on a U Boat
Two more books I'd recommend. Alexander's "Utmost Savagery" about Tarawa and because I saw the old 1943 movie again this morning "Guadalcanal Diary". I have an old first edition of this I read years ago, I'm thinking about breaking it back out!