Girl from the north shares letters with girl from the south in book for the masses (6)
I believe the answer is:
missal
'book for the masses' is the definition.
I can't judge whether this definition defines the answer.
'girl from the north shares letters with girl from the south' is the wordplay.
'girl from the north shares letters' becomes 'mis' (I am not sure about this - if you are sure you should believe this answer much more).
'with' is a charade indicator (letters next to each other).
'girl' becomes 'las' (I've seen this in other clues).
'from the south' says the letters should be written in reverse (in a down clue, letters go up).
'las' written backwards gives 'sal'.
'mis'+'sal'='MISSAL'
'in' is the link.
Can you help me to learn more?
(Other definitions for missal that I've seen before include "Book of prayers and rites" , "book for complete service" , "service manual" , "A book of prayers used in mass" , "Book used in RC Mass" .)