In this post, https://www.torahmusings.com/2013/04/my-nom-de-plume-exposed/ , R' Michael Broyde says that "Finally, [Professor Marc B.] Shapiro informed me that the Aderet published a book anonymously, and included his own haskamah to the book (referring to himself in exalted language)." Does anyone have references to resources with more information about this incident?
The book in question is called Aharit HaShanim, and was indeed published anonymously by Aderet, with his own approbation. It discusses the topic of vidui ma'aser.
See footnote 1a to this blogpost for photos of the title page and Aderet's approbation.
An updated version, without the approbations, can be found on HebrewBooks.org here.
For a further example, the title page of Zekher L'Mikdash indicates anonymous authorship, but it is generally attributed (e.g. by HebrewBooks.org) to Aderet. It contains an approbation from Aderet himself.
See the comments section of this blogpost for excerpts of an article by R. Ari Kahn discussing this case.