After ten years, books degraded (8)
I believe the answer is:
decadent
'degraded' is the definition.
(thesaurus)
'after ten years books' is the wordplay.
'after' is a charade indicator (letters next to each other).
'ten years' becomes 'decade' (a decade is ten years).
'books' becomes 'NT' (abbreviation for New Testament).
'decade' next to 'nt' is 'DECADENT'.
(Other definitions for decadent that I've seen before include "Sybaritic" , "Symbolist" , "Depraved" , "Rotten" , "Culturally in decline" .)