Is that a book on farming?If so,I want to buy ______. (one OR it?)

题目

Is that a book on farming?If so,I want to buy ______. (one OR it?)

答案

it
前面显然是指着一本书说的,要买就买它咯,不会再有泛指买哪堆书里其中随便一本一说的.