The Red Book of Westmarch (sometimes the Thain's Book after its principal version) is a fictional manuscript written by hobbits, related to the author J. R. R. Tolkien's frame stories. It is an instance of the found manuscript conceit, a literary device to explain the source of his legendarium. In the fiction, it is a collection of writings in which the events of The Hobbit and The Lord of the Rings were recounted by their characters, and from which Tolkien supposedly derived these and other works. The name of the book comes from its red leather binding and casing, and from its having been housed in the Westmarch, a region of Middle-earth next to the Shire.
In reality, Tolkien modelled its name on the Red Book of Hergest. By using the conceit of a found manuscript, he was following a tradition in English literature established by Samuel Richardson in the 18th century. He was also attempting, according to the scholar Gergely Nagy, to fit The Lord of the Rings into his presentation of his legendarium as a genuine-seeming collection of tales and myths, by ascribing the documents to the hobbit Bilbo Baggins.
Fictional development
The Red Book of Westmarch is part of Tolkien's framing of The Hobbit as part of a long tradition of manuscripts, which he happened to have found.
There and Back Again
In The Hobbit, Tolkien writes of the protagonist and title character Bilbo Baggins composing his memoirs. Bilbo thinks of calling his work There and Back Again, A Hobbit's Holiday. Tolkien's full name for the novel is indeed The Hobbit or There and Back Again.
In The Lord of the Rings, this record is said to be written in his red leather-bound diary. Bilbo says to Gandalf that his intended ending would be him living "happily ever after to the end of his days". This is in fact a rephrased line from the final chapter of The Hobbit, originally conveyed through third-person narrative voice.
The Downfall of the Lord of the Rings
Bilbo expands his memoirs into a record of the events of The Lord of the Rings, including the exploits of his kinsman Frodo Baggins and others. He leaves the material for Frodo to complete and organize. Frodo writes down the bulk of the final work, using Bilbo's diary and "many pages of loose notes". At the close of Tolkien's main narrative, the work is almost complete, and Frodo leaves the task to his gardener Samwise Gamgee.
In the last chapter of The Return of the King, Tolkien provides a "title page" for the Red Book of Westmarch inscribed with a succession of rejected titles. The final title is Frodo's:
Translations from the Elvish
Bilbo had translated material from Elvish lore from the Elder Days. This work, Translations from the Elvish, by B.B., comprised three volumes, also bound in red leather. After the defeat of Sauron (the Lord of the Rings) Bilbo gives these volumes to Frodo. These four volumes were "probably" (according to Tolkien) kept in a single red case.
Red Book
The volumes then pass into the keeping of Samwise Gamgee, Frodo's servant and later mayor of the Shire. In time, the volumes are left in the care of Sam's eldest daughter, Elanor Fairbairn, and her descendants (the Fairbairns of the Towers or Wardens of Westmarch). A fifth volume containing Hobbit genealogical tables and commentaries is composed and added at unknown dates, presumably over a long period of time, by unknown hands in Westmarch. This collection of writings is collectively called the Red Book of Westmarch.
Thain's Book
Tolkien states that the original Red Book of Westmarch was not preserved, but that several copies, with various notes and later additions, were made. The first copy was made on the request of King Elessar of Arnor and Gondor, and was brought to Gondor by Thain Peregrin I, who had been one of Frodo's companions. This copy was known as The Thain's Book and "contained much that was later omitted or lost". In Gondor it underwent much annotation and correction, particularly regarding Elvish languages. Also added was an short version of The Tale of Aragorn and Arwen by Faramir's grandson Barahir.
The story then runs that a copy of a revised and expanded Thain's Book was made probably by request of Peregrin's great-grandson and delivered to the Shire. It was written by the scribe Findegil and stored at the Took residence in Great Smials. Tolkien says this copy was important because it alone contained the whole of Bilbo's Translations from the Elvish.
This version somehow then survives until Tolkien's time, and he translates the Red Book from the original languages into English and other representative languages or varieties, such as Old English for Rohirric.