A nobleman's book is bound to have one (5)
I believe the answer is:
title
'a nobleman's book' is the definition.
'title' can be an answer for 'book' (I've seen this before). I'm not sure about the 'a nobleman's' bit.
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for title that I've seen before include "Right - name - address" , "Lord, sir or doctor, say" , "Identifying name" , "headline?" , "Proprietorship" .)