Book right on the shelf (6)
I believe the answer is:
ledger
'book' is the definition.
(I know that book can be written as ledger)
'right on the shelf' is the wordplay.
'right' becomes 'r' (common abbreviation).
'on' says to put letters next to each other.
'the shelf' becomes 'ledge' ('ledge' can be a synonym of 'shelf').
'r' after 'ledge' is 'LEDGER'.
(Other definitions for ledger that I've seen before include "Old-fashioned heavy accounts book" , "Accounts journal" , "Book for keeping financial accounts" , "Heath -, Australian actor" , "Elderly resident" .)