I enjoted the book that you told me to read it.改错,说明原因

I enjoyed the book that you told me to read .( 去掉IT，因为引导词代替其提前了）

I enjoted the book that you told me to read.
去掉it。定语从句中引导词that做宾语，不需要it，不然就有两个宾语了

I enjoyed the book that you told me to read it.
去掉最后的it
that you told me to read 是定语从句,修饰先行词book,引导定语从句的关系代词that,在从句中充当read的宾语,所以不需要再加上it作宾语了.