### Clue: Our Mutual Friend", say, which makes the set? (9,6)
### Answer: COMPANION VOLUME 
 ### Explanation: Definition: which makes the set
 Cryptic defn: The novel by Charles Dickens cryptically refered to;say, as a book;volume about a companion;friend.
Defn: That which would complete a set of, say, books. (And this book, the last to be completed by Dickens can be said to complete the set of books authored by him.)