I say! Cryptographer's decrypted registry with these books (9)
I believe the answer is:
apocrypha
'these books' is the definition.
(I've seen this before)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for apocrypha that I've seen before include "religious text" , "Fourteen books of the Old Testament excluded from most versions of of the Bible" , "Doubtfully-authentic writings" , "dropped books" , "In which Judith is" .)