A set of books one is opposed to? (4)
I believe the answer is:
anti
'opposed to?' is the definition.
(anti means opposing something)
'a set of books one' is the wordplay.
'set of books' becomes 'NT' (New Testament).
'one' becomes 'i' (Roman numeral).
'a'+'nt'+'i'='ANTI'
'is' acts as a link.
(Other definitions for anti that I've seen before include "Opposed to followers of Hitler" , "Naysayer" , "Person opposed" , "Adversary" , "Who pooh-poohs all" .)