Is Book a collective noun as it has collection of pages?
Collective nouns are an informal part of language. There are hundreds of standard collective noun, but any noun that suits a situation can function as a collective noun. A collective noun is a function of a noun, not a form of a noun.
The standard use of 'book' as a collective noun is 'a book of stamps' or 'a book of matches'. The standard collective noun for the noun 'book' is 'a library of books'.
A 'book of pages' is a suitable use of the noun 'book' as a collective noun.
The noun 'collection' is a standard collective noun for a collection of stamps (or the plural, collections of stamps), a collection of objects (or things). A collective noun is an informal part of language. The noun 'collection' lends itself very well to many situations where there is no standard collective noun, for example, a collection of ideas, a collection of parts, a collection of bottle caps, etc.
There is no standard collective noun for a group of collectors (of anything). A collective noun is an informal part of speech. When there is no standard collective noun, a noun that suits the situation can be used, for example, a collection of collectors, a conventions of collectors, a clump of collectors, etc.