西界红皮书（Red Book of Westmarch） aka. Red Book of the Periannath is the book in which the legendarium of Middle-earth was written. It is bound in red leather.
The first Red Book was written as a diary by the Hobbit Bilbo Baggins and recounted his quest for Erebor, which he called There and Back Again. He filled it with Elven lore while he retired to Rivendell (Translations from the Elvish) where he gave it to Frodo.
Frodo organized Bilbo's manuscript and used it to write down his own quest during the War of the Ring. Inscribed within, it reads:
The book contained Bilbo's translations of Elven legends from the Elder Days, and various Hobbit poems. A lot of background information on the realms of Arnor, Gondor, and Rohan was added to it by Peregrin Took and Meriadoc Brandybuck from their contacts in Rohan and Gondor. Other material was provided by King Elessar.
After Bilbo and Frodo left for Valinor, the Red Book passed into the keeping of Samwise Gamgee, the Mayor of the Shire. The book was left in the possession of Sam Gamgee's eldest daughter, Elanor Fairbairns, and her descendants (the Fairbairns of the Towers or Warden of Westmarch).
The original book was formed from Bilbo's private diary and attached to it, in a single red case, were three large volumes bound in red leather. There was a fifth volume containing commentaries, genealogies, and various other matter concerning the hobbit members of the Fellowship. Several copies, with various notes and later additions, were made and copies were passed on to future generations, of which one, the "Thain's Book", is the most important.
The "original" version of the Red Book contained the story of Bilbo's journey as it originally stood: thus, Gollum willingly gives the One Ring to Bilbo, and there is no trace of the Ring's hold over Gollum. Later copies of the Red Book contained, as an alternative, also the true account (later written in by Frodo), where Bilbo comes across the Ring by accident.