Old SF Book

I am not sure if I am in the correct place but I have a question. I am looking for the name of a SF Book I read in the late 50’s or early 60’s. I do not recall much about the book. It was one of those after a disaster to the human race books. The main person had a name like IS, IK or ID or I think a two letter name. One think that I recall is that when he was older he was suprised that the younger generation was using pennies to make arrow heads for their bows. Thanks for any help.