One part author's written about that is included in books (6)
I believe the answer is:
moiety
'one part' is the definition.
(I've seen this before)
'author's written about that is included in books' is the wordplay.
'author's' becomes 'my' (I can't explain this - if you can you should believe this answer much more).
'written about' indicates putting letters inside.
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'included in' indicates putting letters inside.
'books' becomes 'OT' (abbreviation for Old Testament).
'ie' put inside 'ot' is 'oiet'.
'my' placed around 'oiet' is 'MOIETY'.
Can you help me to learn more?
(Other definitions for moiety that I've seen before include "One of two equal parts" , "Part" , "A legal sort of half" , "Either of two" , "Oy! Time for a half" .)