Well I think you've been very astute to see, and understand that the book ID mentioned in that image isn't fit for the purpose.
My guess is the tutor either wants you to create a compound ID (out of two Fields) or add your own ID, an autonumber field.
I would guess that the latter, an "Autonumber field", but I have no idea what is in your tutors mind, you need to get that from reviewing your recent lessons and other information provided by the tutor...