Principal transaction included one with books (7)
I believe the answer is:
salient
'principal' is the definition.
(thesaurus)
'transaction included one with books' is the wordplay.
'transaction' becomes 'sale' (sales are a type of transaction).
'included' means one lot of letters goes inside another.
'one' becomes 'I' (Roman numeral).
'with' is a charade indicator (letters next to each other).
'books' becomes 'NT' (abbreviation for New Testament).
'sale' going around 'i' is 'salie'.
'salie'+'nt'='SALIENT'
(Other definitions for salient that I've seen before include "Noteworthy" , "Entails (anag) - most important" , "Leaping (heraldry)" , "part of defences" , "Projecting outwards; significant" .)