What does the cryptic crossword clue "Note change in book for the church (7)" mean?

The wordplay uses a charade (building blocks). PS (synonym="note, at the end of a letter") + ALTER (synonym="ALTER") = PSALTER The answer is PSALTER.

Note change in book for the church (7)

Guardian Cryptic #29285 | 20 Down | 2024-01-22 flower