Yellowskin or the Year-book of Tuckborough was the oldest of the Hobbit chronicle books, said to contain entries dating back some 900 years before the War of the Ring (since about T.A. 2000).
It contained chronicles of the Took family, including marriages, births and deaths, but also legal details and references to important events in the Shire as a whole.
Yellowskin became a source for much of the historical information, especially about the Tooks, to find its way into the Red Book of Westmarch.
The book recorded an archaic form of the Hobbitish language, and from this the archaic names of the weekdays (Sterrendei, Sunnendei etc.) were preserved in the Red Book.
The name 'Yellowskin' presumably derives from the aged and decayed condition of the book, suggesting that the records of the Tooks were recorded on vellum, or some other form of parchment made from skin. The name may have been inspired by that of a thirteenth-century manuscript called Fagrskinna, meaning "fine vellum" in Old Norse. (Fagrskinna was destroyed by fire in the eighteenth century, but not before a copy was made.)
Other versions of the legendarium
In the second version of Appendix F, the official name of the book popularly called Yellowskin was The Great Writ of Tuckborough and it was supposed to be nearly a thousand years old.