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 (Modern English: "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 J.R.R. 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.