Keep very good set of books penned by master (8)
I believe the answer is:
maintain
'keep' is the definition.
(I know that keep can be written as maintain)
'very good set of books penned by master' is the wordplay.
'very good' becomes 'AI' (resembles 'Am', old-fashioned term for excellent).
'set of books' becomes 'NT' (New Testament).
'penned' is an insertion indicator (inserted letters are enclosed or penned in).
'by master' becomes 'main' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ai'+'nt'='aint'
'aint' going within 'main' is 'MAINTAIN'.
Can you help me to learn more?
(Other definitions for maintain that I've seen before include "Keep in good order" , "declare" , "Keep in existence or in repair" , "Continue with an opinion" , "Keep up - carry on" .)