Book of the Foxrook
It consists of a 16-page notebook containing "the key to a secret code consisting of a rune-like phonetic alphabet and a sizable number of ideographic symbols". The invented script is called Privata Kodo Skauta ("Private Scout Code"). The book also contains comments on Esperanto.
The existence of this manuscript was apparently first mentioned by Humphrey Carpenter in his J.R.R. Tolkien: A Biography (p. 37). A page from the Book of the Foxrook was reproduced in the exhibition catalogue Tolkien: Life and Legend. An analysis and transcription of the same page appeared in the article "Tolkien and Esperanto" (cf. pp. 29-34), published in SEVEN, Volume 17.