Rider is one for the books (8)

I believe the answer is:

addendum

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'rider' is the definition.
(thesaurus)

I don't understand the rest of the clue.

(Other definitions for addendum that I've seen before include "Something attached to the end, say of book" , "Addition at end of book" , "Supplementary text tacked on to the end" , "Extra item added at end of book" , "PS" .)

I've seen this clue in the Irish Times.

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