Aesa

I could definitely see there being some other ancient singer text like the Eila Stele that either wasn't translated yet or will be discovered during RoW. Then again, what if Rhythm of War is the Eila Stele? We don't know if the bit we've seen is the entire thing or just a key fragment, and it's a title that kind of fits considering how it's concerning the very start of the war between singer and human. There's even some more Way of Kings epigraphs in Oathbringer right there alongside it, so it wouldn't even be the only time a given in-universe book has been quoted outside of its namesake.

The names of the books are all those of in-universe books. It would be weird for that naming pattern to suddenly change, though I imagine the book is named after one of the singer rhythms. I'm personally thinking it'll be some sort of ancient singer or listener book that Venli happens across somehow, just because another book that's being written by a main character concurrent to the events of the series itself like Oathbringer feels a little strange.