Siblings of the Firstborn?

[Galin]

Interestingly, Tolkien once noted: '... but the lore of the Eldar did not depend on perishable records, being stored in the vast houses of their minds (*23). When the Eldar made records in written form, even those that to us would seem voluminous, they did only summarise, as it were, for the use of others whose lore was maybe in other fields of knowledge, (*24) matters which were kept forever undimmed in intricate detail in their minds.'

Note 24: 'And as some insurance against their own death. For books were made only in strong places at a time when death in battle was likely to befall any of the Eldar, but it was not yet believed that Morgoth could ever capture or destroy their fortresses.'

The Note on the Shire Records was added in the 1960s, after Tolkien had started to think of the Numenorean line of transmission, and already with the publication of The Adventures of Tom Bombadil, the Numenorean factor had appeared in print: 'No. 14 also depends on the lore of Rivendell, Elvish and Númenorean, concerning the heroic days at the end of the First Age; it seems to contain echoes of the Númenorean tale of Turin and Mim the Dwarf.'

So I think Imladris must contain some Elvish lore -- not just simply lore written in Elvish -- but how much purely Elvish written lore survived in Imladris? so that Bilbo could translate it 'for Tolkien', so to speak.

Surely some of the Elvish lore could be living lore, as in the memory of certain Elves, but the Silmarillion 'proper' (as a text) was not to be Elvish according to Tolkien's later ideas, and the Silmarillion account of the three ambassadors derived from memory in the first place at least, as Rumil had not invented writing until after this event.