Edit: I think it's "Secrets of a Baby Nurse," by Marsha Podd. (Maybe. It's been a long time.) I hope that's it because so many of those books were uninformative or simply reassuring (e.g. "Yes, it's normal ..."). This one talks about how your baby is communicating with you from the start and how to watch for that to make both of your lives easier.
source: am parent of more than one child, each of whom had distinct personalities from birth
I love my 2 month old but her personality mainly seems to be "I enjoy drinking milk and sleeping"
I won’t definitively state newborns don’t have personalities, but my unscientific hunch is sleep-deprived parents are simply projecting and seeing what isn’t really there.
I love both my children, but for the first six weeks they are simply base creatures, outwardly.
The oldest was pretty lazy and quiet during the day, but did get bouncy when my wife tried to sleep. The combination of couch potato and getting active when stimuli stops and the boredom hits is present in the kid to this day.
Middle child was super-active the whole way through labor and came out active. Was surfing furniture standing up at eight months.
I could go on at length.
Main point is - we saw the differences before they were born.
Of course, humans are still a different species and our offspring have a much more complex ontogenesis and so I would venture and say our individuality development is even more sensitive to pre-natal environmental, post-natal and genetic factor changes.
But ultimately, we are all animals here so we are governed by very similar broad rules.