We invented tools to allow us to apply knowledge in domains where we humans have limited capability. Rulers remember sizes for us, because we don t do that very accurately or very consistently. Saws cut wood or metal for us, because our hands are physically incapable of doing that. Hammers deliver a hard impact of high momentum on a small area that would cause us a lot of pain were we to try it without using that particular device. So hardware is physically encapsulated knowledge that has been constructed to allow us to apply a particular type of knowledge, in a limited and specific domain.
Books exist to transport knowledge from one place to another.
This transportation can be through space or through time. We keep diaries because we forget. Books made possible the modern world. The oral tradition of the human race is precisely that the passing of essential knowledge from one generation to the next. Given the rather porous nature of the human memory, each generation had mostly to develop each idea anew.