Relish good book, cracking open bitter (6)
I believe the answer is:
savour
'relish' is the definition.
(synonyms)
'good book cracking open bitter' is the wordplay.
'good book' becomes 'AV' (Authorised Version of the Bible).
'cracking open' indicates putting letters inside (inserted letters crack into the word).
'bitter' becomes 'sour' (I've seen this before).
'av' placed into 'sour' is 'SAVOUR'.
(Other definitions for savour that I've seen before include "Enjoy taste of (food)" , "Taste or smell something - yummy" , "Delight in" , "Taste, relish" , "Characteristic taste" .)