Volume of sanctimonious religious books (4)
I believe the answer is:
pint
'volume' is the definition.
'sanctimonious religious books' is the wordplay.
'sanctimonious' becomes 'pi' (old-fashioned term for pious).
'religious books' becomes 'NT' (New Testament).
'pi'+'nt'='PINT'
'of' acts as a link.
(Other definitions for pint that I've seen before include "Unit of capacity" , "The working man's beer measure" , "Standard measure of Guinness" , "Liquid measure" , "Half a quart" .)