Rider changes books of psalms (8)
I believe the answer is:
psalters
'books of psalms' is the definition.
(I've seen this before)
'rider changes' is the wordplay.
'rider' becomes 'PS' (a rider is a clause added to a contract, so a P.S.).
'changes' becomes 'alters' ('alter' can be a synonym of 'change').
'ps'+'alters'='PSALTERS'
(Other definitions for psalters that I've seen before include "religious books" , "Versions of the Book of Psalms" , "liturgical books" , "Books of Psalms" .)