Hi jno - apparently your idea
jno his book became jno's book - was common in the C18 but is not what happened.
and that was because Emma her book, also became Emma's book...
Chaucer sometimes uses the old genitive, the Kingis grace springs to mind, where as we would say, the King's grace.
SInce the old anglo saxon has possessives, his her that agreed with the possessed, as we learn in French, and English has possessives that agree with the possessor, he or she what has got it, there must have been a period when anglo saxon passed through having unified genders (or no gender) but that period has not been identified.