Excellent book? Exactly! (5)

I believe the answer is:

plumb

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'exactly' is the definition.
(plumb can mean exactly or precisely)

'excellent book?' is the wordplay.
'excellent' becomes 'plum' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'book?' becomes 'b' (abbreviation for book).
'plum'+'b'='PLUMB'

Can you help me to learn more?

(Other definitions for plumb that I've seen before include "Dead" , "Check depth of water below" , "Fathom - out-and-out" , "Lead weight for measuring depth" , "Absolutely straight - sound" .)

I've seen this clue in The Sun.

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