Book ultimately unfinished (5)
I believe the answer is:
atlas
'book' is the definition.
(atlas is a kind of book)
'ultimately unfinished' is the wordplay.
'ultimately' becomes 'atlast' (ultimately means lastly).
'unfinished' means to remove the last letter.
'atlast' with its final letter taken away is 'ATLAS'.
(Other definitions for atlas that I've seen before include "In Greek myth, a Titan made to support the heavens" , "Segment of spine" , "N. African mountains; a Titan" , "Titan compelled by Zeus to support the sky" , "One of the Titans" .)