Nancy's Mysterious Letter is the eighth volume in the Nancy Drew Mystery Stories series. It was first published in 1932 and was penned by Walter Karig, a replacement writer for Mildred Wirt Benson. Benson declined series work when the Depression forced a reduction in the contract fee provided to Stratemeyer Syndicate writers, so Karig, already an established Stratemeyer writer, took over the authorship. Due to Karig having died in 1956, the 1932 version passed into the public domain in Canada and other countries that have a life plus 50 policy, in 2007.

The Nancy Drew Mystery Stories is the long-running "main" series of the Nancy Drew franchise, which was published under the pseudonym Carolyn Keene. There are 175 novels — plus 34 revised stories — that were published between 1930 and 2003 under the banner; Grosset & Dunlap published the first 56, and 34 revised stories, while Simon & Schuster published the series beginning with volume 57.