Book ultimately missing conclusion (5)

I believe the answer is:

atlas

'book' is the definition.
(atlas is a kind of book)

'ultimately missing conclusion' is the wordplay.
'ultimately' becomes 'atlast' ('at last' can be a synonym of 'ultimately').
'missing conclusion' means to remove the last letter.
'atlast' with its last letter taken away is 'ATLAS'.

(Other definitions for atlas that I've seen before include "Titan who supported the heavens" , "God" , "Asian satin" , "Collection of maps" , "Book of geography" .)

I've seen this clue in The Telegraph.

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