One won't be able to understand characters in a book (9)

I believe the answer is:

nonreader

This clue could be a double definition.

'one won't be able to understand characters' is the first definition.
(not understanding characters ie letters)

'a book' is the second definition.
Both the definition and answer are singular nouns.
Perhaps they are linked in a way I don't understand?

'in' is the link.

I've seen this clue in The Times.

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