Book that's finally inconclusive (5)
I believe the answer is:
atlas
'book' is the definition.
(atlas is a kind of book)
'finally inconclusive' is the wordplay.
'finally' becomes 'atlast' ('at last' can be a synonym of 'finally').
'inconclusive' means to remove the last letter.
'atlast' with its final letter taken off is 'ATLAS'.
'that's' acts as a link.
(Other definitions for atlas that I've seen before include "Plans" , "The first cervical vertebra" , "used for maps" , "Size of paper once" , "Mythological Titan (involved with the world of maps?)" .)