A better book? (6)

I believe the answer is:

primer

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This clue might be a double definition.

'a better' is the first definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

'book?' is the second definition.
(I know that primer is a type of school text)

Can you help me to learn more?

(Other definitions for primer that I've seen before include "it might be used by one 7 [PAINTING]" , "introduction" , "Elementary text book" , "device for detonating charge" , "First coat - book" .)

I've seen this clue in the Evening Standard.

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