Five books the writer's given as promised (6)
I believe the answer is:
votive
'promised' is the definition.
Both the definition and answer are adjectives. Maybe you can see an association between them that I can't see?
'five books the writer's given' is the wordplay.
'five' becomes 'V' (Roman numeral).
'books' becomes 'OT' (Old Testament).
'the writer's' becomes 'ive' ('I have' can refer to the person writing).
'given' is a charade indicator (letters next to each other).
'v'+'ot'+'ive'='VOTIVE'
'as' is the link.
Can you help me to learn more?
(Other definitions for votive that I've seen before include "Offered to fulfil vow" , "Offered in fulfilment of a vow" , "offered as promised" , "Given in fulfilment of vow" , "dedicated" .)