The most common and most likely answer to this crossword clue is the 7 letter word PSALTER
I've seen this crossword clue in The Guardian
P
S
A
L
T
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More clues leading to the result PSALTER
- Wit in a devotional book
- A belated idea to edit book of David’s songs
- And another thing: change liturgical book
- Ancient mariner goes in for each song book
- Last-minute note to change church book
- A small weak brandy drunk polishing off book? Non-sense!
- Appendix to amend in sacred work
- There’s talk of someone preserving a book of holy songs
- Footnote: change volume in church
- Book Alps resort and model with some excitement before end of October
Buddy explains!

The straight part
We know that "church book" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case church book = psalter.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does PSALTER mean?
PSALTER noun- The Book of Psalms. Often applied to a book containing the Psalms separately printed.
- Specifically for Anglicans, the Book of Common Prayer which contains the Book of Psalms. For Catholics, the Breviary containing the Psalms arranged for each day of the week.
- In the Roman Catholic Church, a rosary consisting of one hundred and fifty beads, corresponding to the number of the Psalms.
- (obsolete or rare) psaltery (the musical instrument).