Book ultimately ending too soon (5)
I believe the answer is:
atlas
'book' is the definition.
(I know that atlas is a type of book)
'ultimately ending too soon' is the wordplay.
'ultimately' becomes 'atlast' (ultimately means lastly).
'ending too soon' means to remove the last letter.
'atlast' with its final letter removed is 'ATLAS'.
(Other definitions for atlas that I've seen before include "Titan who bore the sky on his shoulders" , "A geog. book; N. African range" , "It appears at the top of the spine" , "Titan who held up the sky" , "Segment of spine" .)