How do you footnote a footnote from another book?

To footnote a footnote from another book, you should first cite the original source of the footnote in your text. Then, in your footnote, indicate that you are referencing another author's footnote. You can format it as follows: “As noted in [Author's Name], [Title of the Book], footnote [number].” Be sure to include full citation details for both the original source and the source you are citing.