Junk mail included covers of sales book (6)
I believe the answer is:
missal
'book' is the definition.
(I know that missal is a type of liturgical book)
'junk mail included covers of sales' is the wordplay.
'junk' is an anagram indicator.
'included' means one lot of letters goes inside another.
'covers of' means to remove the middle letters.
'sales' with its centre taken out is 'ss'.
'mail' with letters rearranged gives 'mial'.
'mial' going around 'ss' is 'MISSAL'.
(Other definitions for missal that I've seen before include "Prayer book for th Catholic mass" , "Catholic mass text book" , "Book of prayers and rites" , "book needed to get through service" , "Yearbook" .)