Best-seller religiously read (5)

I believe the answer is:

bible

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'read' is the definition.
The definition and answer can be both related to communication as well as being singular nouns.
Perhaps they are linked in a way I don't understand?

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for bible that I've seen before include "Book for someone swearing?" , "Holy Writ" , "See 12" , "Book of the Scriptures" , "Christian holy book" .)

I've seen this clue in the Evening Standard.

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