Book of Hidden Things

A book referenced by Claes van der Heyl, who had visited "Yian-Ho, that lost and forbidden city of countless aeons" and apparently awakened something while he was there. Van der Heyl left a Latin message saying "That which I have awaked and borne away with me, I may not part with again. So is it written in the Book of Hidden Things." [HPL Diary (online text)]