Ashi no Himitsu

The Ashi no Himitsu is an ancient Foot Clan book in the IDW continuity. It serves as a comprehensive history of the clan's early history, including extensive information about the life of its founder Takeshi Tatsuo, the politics that the Foot Clan was involved in, and even about the sinister magics that were worked by the witch Kitsune. Furthermore, it has a great deal of information pertinent to the Dragon Warrior.

In the present, Oroku Saki had a partially-intact copy of the Ashi no Himitsu in his possession, but there were many pages missing from it. Other pages had been found by Dr. Miller, an expert on the Foot Clan's history, who had uncovered the partial text in an old temple.

The Turtles managed to abscond with Shredder's copy of the text, and learned a great deal from it, including how Shredder had managed to become immortal. Once they had finished with the book, Splinter burned it.