Get on with books representative (5)
I believe the answer is:
agent
'representative' is the definition.
(agent is a kind of representative)
'get on with books' is the wordplay.
'get on' becomes 'age' (synonyms).
'with' says to put letters next to each other.
'books' becomes 'NT' (New Testament).
'age'+'nt'='AGENT'
(Other definitions for agent that I've seen before include "Author" , "Traveller" , "Negotiator" , "Representative - means" , "Spy; representative" .)