Citing the notes of an author, included in an afterword written by someone else?

Factual Questions
1

I’m writing an essay on a book. I’d like to make use of a point the author made in his notes, which are included in the book’s afterword (though the afterword was written by someone else). I’m doing the cites parenthetically, and I’d like it to be clear that the source of the point is the book’s author, and not the afterword’s.

How do I do this?

2

I guess what I’m really asking is: how do I parenthetically cite Person A’s quote of Person B while making it clear that the quote came from Person B?

3

(Exapno Mapcase, as quoted in “Afterward” by Absolute in Exapno Mapcase’s GQ Questions for Dummies.)