Reserve on all sides today (4)

I believe the answer is:

book

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'reserve' is the definition.
(booking is a kind of reserving)

I don't understand the rest of the clue.

(Other definitions for book that I've seen before include "Reserve - bets taken on a race" , "Paperback" , "Leaves, bound" , "Reserve - something to read" , "Reserve; tome" .)

I've seen this clue in the Irish Times.

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