Here's why:
* It names a thing: A book is an object that contains written or printed material.
* It can be the subject of a sentence: "The book is on the table."
* It can be the object of a verb: "I read the book."
However, "book" can occasionally function as an adjective when it modifies another noun. For example:
* "Book store": Here, "book" describes the type of store (a store that sells books).
So, while "book" is most often a noun, it can take on the role of an adjective in certain contexts.