A photograph of the endpapers at the front of a copy of Doyle's "The Sign of the Four", sitting on a green cutting mat. The endpapers depict a map of the Red Fort of Agra, a plot point described in the text.
https://files.mastodon.social/media_attachments/files/113/734/329/597/038/640/original/2d64ace0db016a18.jpg