The Steven Ambrose book is "okay", but certainly not the best choice as a guide. I suggest that you obtain the following:
-"Stand Where They Fought: A Guide to 150 Normandy Battlefield Sites" by Carlton Joyce
-"Major and Mrs. Holt's Battlefield Guide to the Normandy Landing Beaches"----I would suggest, additionally, that you purchase both the Holt book and map.
-Michelin Map #231 of Normandie, France
At this time of year, be prepared for cold weather, including rain and, possibly, some snow and/or slush. You will also find that some of the more prominent museums will be closed----have fun anyway. I don't know how long you are planning to spend in the area, however, with over 500 memorials, museums, batteries, bunkers and cemeteries----you could spend 4 or 5 days in the area and still not get bored.
Be forewarned----visiting the American Cemetery at St. Laurent, overlooking Omaha Beach, is a very emotionally moving experience.