There is certainly more to the Elven texts than you have seen so far, and more excerpts will be appearing as chapter epigrams in forthcoming books. None of these books exists as a complete manuscript, but that is not to say that it couldn't happen.
As for "The Song of Amrethion", it does not exist in its entirety. As the full poem cycle contains significantly more words than the entire unabridged Oxford English Dictionary, it is something that is unlikely to ever exist as anything more than various excerpts...
Re: Texts and Lore
As for "The Song of Amrethion", it does not exist in its entirety. As the full poem cycle contains significantly more words than the entire unabridged Oxford English Dictionary, it is something that is unlikely to ever exist as anything more than various excerpts...