Whose book is this?It is mine.这句话哪里错了？

whose book is it ?it is mine .
whose book is this ?this is mine .这样懂吗。

真！99.9％的美国人会认为这句话没错。不信你用Google搜索一下“Whose book is this?It is mine.”就明白了

It is my.（因为问句中用whose book“谁的书”，回答就是my book“book省略，避免重复”）

Who's book is this? It is mine.

若问句是whose book is it? 用 it's mine.回答
若问句是Whose book is this? 用 this is my book.回答

没错