Software tools
Open Office Writer: For the creation of the rulebook
Open Office Calc: For odds calculation and combat simulation etc.
Open Office Draw: For schematics and drawings
Open Office Base: For maintaining the reference bibliography
(Yeah, I'm an O³ fanboy)
Gimp: for photo editing and creation of illustrations

Heimdal Series of Special Editions on D-Day - Various Authors
Band of Brothers - Stephen Ambrose
D-Day - Stephen Ambrose
Beyond Band of Brothers - Dick Winters
Bravery in Battle - David Eshel 
The D-Day companion - Jane Penrose

Fact Finding
Osprey Guides on more or less everything relevant for the Normandy Theatre and equipment