Declining period of time with books (8)
I believe the answer is:
decadent
'declining' is the definition.
(I've seen this before)
'period of time with books' is the wordplay.
'period of time' becomes 'decade' (decade is a kind of period of time).
'with' is a charade indicator (letters next to each other).
'books' becomes 'NT' (New Testament).
'decade'+'nt'='DECADENT'
(Other definitions for decadent that I've seen before include "Reflecting a state of moral decline" , "abandoned" , "Reprobate" , "Self-indulgent or immoral" , "Morally corrupt" .)