
The book of the bitch was written a long time ago by a very experienced dog breeder. Breeding dogs isn't a science based skill, no amount of research & experiment/studies can cover every eventuality.
If you ask a vet how long it takes for bitch to whelp, they will give you a scientific answer, however bitches don't all conform to science, a friend of mine had a bitch that whelped 7(seven)puppies in 70 minutes, she never appeared to go through the early stages of whelping as she was fast asleep laid next to my friend, she woke up & delivered the first pup within 5 mins. Clever bitch popped into her whelping box to deliver the litter & when she had finished & had been cleaned up, she happily nursed her puppies. My friend's vet was amazed at the speed of the whelping, her daughter & grand daughter were equally speedy giving birth.