If there's only time for ONE book: The Bedford Boys by  Alex Kershaw.  An
emotional memoir of the first wave of men who landed on Omaha Beach.

If there's only room for  ONE guidebook: DDay by Stephen Ambrose. A
readable account based on  4000+ eyewitness interviews, covering planning to
the initial post-invasion period. Helpful for reference while visiting sites.

Citizen Soldiers, by Stephen Ambrose.  Only the first few chapters deal with D-
Day, but this bestseller explains overall Allied leadership and planning.

Voices of Dday, by Ronald Drez.  Excerpts from the 4000+ interviews arranged
in chronological order by subject.  Adds a human perspective to each place; best
read in small bits.

Saving Private Ryan, by Max Collins. A quickie read written from the movie
screenplay. This surprisingly moving paperback is more intense than the movie.

Parachute Infantry: An American Paratrooper's Memoir of D-Day and the Fall
of the Third Reich
, by David Webster.  A personal account by a college-
educated enlisted man, one of the Band of Brothers.  This book presents a less
sentimental view of military life and unit leadership than Ambrose
's account.

One Man's War: A Memoir of WW II, by Harold Gordon. A well-written and
witty account by a soldier who landed on DDay +1. The  description of physical
and emotional responses to combat are memorable.

Band of Brothers, by Stephen Ambrose.  This fine book was the original basis
for the HBO miniseries. Ambrose follows a group of 101st Airborne troops from
their pre-war training to Munich.

The Biggest Brother: The Life of Major Dick Winters, The Man Who Led the
Band of Brothers
by Larry Alexander.  An authorized biography of the man
made famous in Band of Brothers.  This book answers  
"what happened then?"
in regards to Easy Company of the 506th.

Eyewitness on Omaha Beach by Dr. Harold Baumgarten. A memoir of DDay
from a 29th Infantry soldier who was seriously wounded.  Dr Baumgarten
's
memories formed the opening scene in Saving Private Ryan, and he assisted  
Stephen Spielberg and Tom Hanks with the movie.

The 25 Best World War II Sites, European Theater by Chuck Thompson.
Three chapters cover the Normandy invasion and breakout. Excellent
information, basic maps, reading recommendations.

The DDay Experience by Richard Holmes. A beautiful book featuring
facsimilies of  original source documents and maps along with  lots of photos.

Beyond the Beachhead: The 29th Infantry Division in Normandy by Joseph
Balkoski. An account of the initial waves of 29th Infantry men on Omaha Beach.
DDay Reading Recommendations
In addition to these personally recommended books, the Omaha Beach
Memorial
site has an excellent page on DDay background reading.  
Check it out for some of the more obscure selections.