In Tolkien's very earliest stories dating from 1917, now published as The Book of Lost Tales volumes one and two, framing device was a mariner called Eriol who found his way to Elfland, and heard the stories sung and recited in The Room of the Log Fire, in The Cottage of Lost Play.
This Eriol was therefore the link between the ancient legendary or mythic world - and the modern world; and over the next decades Eriol became variously re-named and transmuted through AElfwine in the early versions of The Silmarillion in the 1920s and 30s, through the Lost Road fragment of 1936 and the Arundel 'Arry' Lowtham character of the Notion Club Papers of 1945-6 - all of whom were mariners who reached Elfland (Tol Erresea) and brought the ancient legends back to Middle Earth (i.e. the British Isles).
But in the end, it was Bilbo Baggins who did this job - as described in the Prologue: concerning Hobbit to the The Lord of the Rings ^
So Eriol = Aelfwine = Arundel = Bilbo.
Cottage of Lost Play = Elrond's House in Rivendell
Room with the Log Fire = Hall of Fire
^By the end of the first century of the Fourth Age there were already to be found in the Shire several libraries that contained many historical books and records.
The largest of these collections were probably at Undertowers, at Great Smials, and at Brandy Hall. This account of the end of the 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 Fairbairns, 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. 1420-1 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.
The original Red Book has not been preserved, but many copies were made, especially of the first volume, for the use of the descendants of the children of Master Samwise. The most important copy, however, has a different history. It was kept at Great Smials, but it was written in Gondor, probably at the request of the great-grandson of Peregrin, and completed in S.R. 1592 (F.A. 172). Its southern scribe appended this note: Findegil, King's Writer, finished this work in IV 172. It is an exact copy in all details of the Thain's Book in Minas Tirith. That book was a copy, made at the request of King Elessar, of the Red Book of the Periannath, and was brought to him by the Thain Peregrin when he retired to Gondor in IV 64.