Reference book eventually incomplete (5)
I believe the answer is:
atlas
'reference book' is the definition.
(atlas is a kind of reference book)
'eventually incomplete' is the wordplay.
'eventually' becomes 'at last' (at long last).
'incomplete' means to remove the last letter.
'atlast' with its final letter taken off is 'ATLAS'.
(Other definitions for atlas that I've seen before include "Supporter of the classics" , "book of world records?" , "Book of geography" , "Large-format book" , "Titan who bore the sky on his shoulders" .)