I have a bag with books in it.(“in it” 不能省略)?

问题描述:

I have a bag with books in it.(“in it” 不能省略)?
书上说in it 不能省略?为什么?in it 是修饰book的吗?
with books in it 应该是修饰bag的定语吧?不太懂!

I have a bag with books in it.我有一个里面有书的包.
I have a bag with books我有一个带书的包.