Opposed to books about sloth? On the contrary (4)
I believe the answer is:
anti
'opposed to' is the definition.
(I've seen this before)
'books about sloth? on the contrary' is the wordplay.
'books' becomes 'nt' (New Testament).
'about' means one lot of letters goes inside another.
'sloth? on the contrary' becomes 'ai' (I am not sure about this - if you are sure you should believe this answer much more).
'nt' put inside 'ai' is 'ANTI'.
(Other definitions for anti that I've seen before include "Against, informally" , "Negative vote" , "Prefix meaniing against" , "'No' voter" , "(Person who is) not in favour" .)