What is the name of the fictional book said to contain Bilbo’s and Frodo’s accounts in Tolkien’s legendarium?

The fictional book containing Bilbo’s and Frodo’s accounts is The Red Book of Westmarch.

Within J.R.R. Tolkien’s mythology, the Red Book is a collection of writings attributed primarily to Bilbo Baggins and Frodo Baggins. It is presented as an important source behind the narratives that readers know as The Hobbit and The Lord of the Rings. Samwise Gamgee and later descendants are also associated with additions or preservation of the record.

The book’s title refers to its red binding and to the Westmarch, a region associated with the Shire. Tolkien used this fictional-document idea to give the stories an imagined history of transmission, translation, and editing rather than presenting them as ordinary modern narration.

The conceit also helps explain differences in perspective and language. Tolkien often framed himself as a translator or editor of older records, including material connected with Westron and the languages of Middle-earth. The Red Book should not be confused with the Book of Mazarbul, a separate record found in Moria.