Whose is an adjective in the sentence as it is talking about the book. It is asking about the belongingness of the book which is a noun.
An adjective is the one enhancing the meaning of the noun. Thus, whose becomes and adjective.
An adjective is a word that describes a noun. In this case, the word 'whose' describes the noun 'book' by asking it a question. Adjectives that ask question are known as Interrogative Adjective. So, whose is an interrogative adjective.
Nisha Bora answered this
because it is telling more about the book.as the adjective means to describe the noun . as we can see in these sentence the word WHOSE tells more about book.