Crossword Genius

They're bound to be found in reserves (5)

Ross

I believe the answer is:

books

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'reserves' is the definition.
(I know this)

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for books that I've seen before include "Reserve tickets" , "Tomes, volumes" , "See 1" , "Reserves" .)

I've seen this clue in the Irish Times (Crosaire) and The Irish Times.
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