What is the name of the fictional historical book that contains much of The Lord of the Rings?

The fictional historical book containing much of The Lord of the Rings is The Red Book of Westmarch.

Within Tolkien’s mythology, the Red Book is a collection of writings begun by Bilbo Baggins and continued by Frodo and Samwise Gamgee. It records the journeys and events that form the basis of The Hobbit and The Lord of the Rings.

The title refers to the book’s red covering and its association with the Westmarch, a region of the Shire. Tolkien uses this imagined source-book device to give the narratives the appearance of translated or preserved history rather than a story simply told by an outside narrator.

The fictional manuscript also helps explain the relationship between Bilbo’s account, Frodo’s record and later additions. It should not be confused with the Book of Mazarbul, a separate record found in Moria and associated with Balin’s ill-fated colony.