Reference book about time isn't finished (5)

I believe the answer is:

atlas

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

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for atlas that I've seen before include "African mountain range" , "Cartographer's work" , "Collection of maps" , "Book of geography" , "A geog. book; N. African range" .)

I've seen this clue in the Irish Times.

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