With regard to the reading list, I would recommend both 1066 and D-Day, June 6th by Stephen Ambrose. Certainly, 1066 is wonderful if you (or the group) will be visiting the Bayeux tapestry in Bayeux (obviously) or would like a scholarly account of the Norman Conquest. As for the tapestry, it is a marvel and should not be missed if you have time in Bayeux (not sure if the group tour will visit there for any length) and that type of interest. As I recall, it is a timed ticket, so you will not spend a large part of your day.
Also, the Stephen Ambrose book should not be missed before visiting the landing beaches, etc. The website lists the D-Day, June 6th book, but Band of Brothers is also wonderful from the paratrooper perspective. If you watched the mini-series, you will recall that the Normandy invasion began the Companys journey on the Continent. Both are easily read by those who want a general overview of events.
Finally, I have a fun book suggestion for Paris for those who want a light, breezy read - - The Sweet Life in Paris by David Lebovitz, an American pastry chef living in Paris. It is part tale of adaptive, ex pat living and part recipe book. It can be irreverent at times so it may not be suitable for all tastes. It is especially suited for those who have an interest in cuisine. Right up my alley!