Additional material transforms liturgical books (8)

I believe the answer is:

psalters

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'liturgical books' is the definition.
(I've seen this before)

I can't explain the rest of the clue.

(Other definitions for psalters that I've seen before include "Books of Psalms" , "liturgical books" , "religious books" , "Versions of the Book of Psalms" .)

I've seen this clue in The Independent.

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