Legendary knight's useful book (8,6)
I believe the answer is:
tristram shandy
'book' is the definition.
(18th-century novel by Laurence Sterne)
'legendary knight's useful' is the wordplay.
'legendary knight's' becomes 'tristram's' (knight in medieval legends).
'useful' becomes 'handy' (handy can mean useful or convenient).
'tristrams'+'handy'='TRISTRAM SHANDY'
(Other definitions for tristram shandy that I've seen before include "Book" , "Laurence Sterne's 'Gentleman'" , "novel" , "Gentleman booked" , "whimsical hero?" .)