My girlfriend is not a native English speaker, so sometimes she has questions about expressions for which I do not know the origins. This one, in particular, got me curious:
"I have this book" vs. "I have a copy of this book"
About music, you don't say:
"I have a copy of this song" but "I have this song"
Similarly, I never hear:
"I have a copy of this DVD" but "I have this DVD"
However, for books, I have commonly heard, and used myself for no special reason: "I have a copy of this book"
Is there a reason we sometimes refer to a book as a copy of the book? But