Operation Dissimulation: WW2'S Biggest Deception Campaigns
history.co.uk
history.co.uk
I highly recommend any of Ben Macintyre’s books as they’re hugely accessible.
By the time D-Day happened, Nazi Germany was already being relentlessly ground down by the Soviets. I think something like 90% of German casualties occurred along the Eastern front.
Without D-Day, I think the most likely outcome would have been the Soviet occupation of all of continental Europe.
IIRC it was roughly 3-to-1, or 75%