本 is a measure word, whereas 书 means book. Chinese is full of different measure words to refer to different types of objects, a bit like in English we talk about "a cup of coffee" or "a sheet of paper". In these cases, cup and sheet are both the measure words. Chinese is stricter about their use, though, in that you cannot refer to things without their measure words, but you can sometimes refer to them only by their words.
Again, something similar happens sometimes in English. Consider the following dialogue:
"Would you like a cup of tea?"
"Yes, I'd like a cup."
In the reply, the kind of cup we're talking about is understood even though you don't actually mention tea.
Have a look at the following resources:
https://resources.allsetlearning.com/chinese/grammar/Measure_word
https://en.wikipedia.org/wiki/List_of_Chinese_classifiers
https://www.youtube.com/watch?v=KIda0ll6ErM
