The main clause here is The man autographed copies of it at the bookstore. This means that the subordinate clause is what is left - who wrote this new and popular book.
The subordinate clause functions as an adjective, which means that it describes the subject <em>the man. </em>
There are at least two errors in the sentence.
1. The word separate is spelled wrongly. It should read <em>separate</em> and not seperate.
2. There is no period at the end of the sentence.
The sentence should not be written in capital letters only either.
