The Red Book Of Westmarch

[Boromir88]

Ok let's look at the quote we get in the prologue, at the very beginning.
Further information will also be found in the selection from the Red Book of Westmarch that has already been published, under the title of The Hobbit. That story was derived from the ealier chapters of the Red Book, composed by Bilbo himself, and called by him There and Back again.

This is saying The Hobbit and There and Back again is the same story, and are found in the early chapters of the Red Book.

Let's move on.

This account at the end of hte Third Age is drawn mainly from the Red Book of Westmarch. That most important source for the history of the War of the Ring was so called because it was long preserved at Undertowers, the home of the Fairbains, Wardens of the Westmarch. It was in origin Bilbo's private diary, which he took with him to Rivendell. Frodo brought it back to the Shire, together with many loose leaves of notes, and during S.R. 1420-I he nearly filled its pages with his account of the War. But annexed to it and preserved with it, probably in a single red case, were the three large volumes bound in red leather, that Bilbo gave to him as a parting gift. To these four volumes there was added in Westmarch a fifth containing commentaries, genealogies, and various other matter concerning the Hobbit members of the Fellowship.

So the "final" copy of the Red Book has 5 volumes, 3 of which are Bilbos (a part of it being his tale There and Back Again), 1 being the genealogies, commentary...etc, and one being what Frodo had written. When it says "But annexed to it and preserved with it," that is talking about Frodo's volume, he "annexed," or "combined" Bilbo's three volumes to it, and then later the fifth volume was added.

P.S. Welcome to the downs snowbird .