The lore subreddit is in somewhat agreement that it is Savatûn and all of this speculation on "who could it be" is exactly what she wants. Her sole purpose is trickery, and that's what she's done with Truth to Power. For me, a solid piece of evidence is the final word in the whole book, "Aiat". This word has only been shown to us in the Book of Sorrows which was written by Oryx.
The longer we speculate and investigate this book, the stronger Savatûn will get.
<Quria is just as likely. Quria has part of its will intact, just enough to tell us things. While Savathûn is likely, a lot of evidence points to Quria.
If it's Quria, then Savathûn hasn't found the Distributary, making her Imbaru plan null, meaning speculation about the messages won't be empowering her.
The answer to the Dreaming City problem is incredibly simple, the book even says that. Both Savathûn and Quria are the simplest answers.
The glitch in the text, the misunderstanding about the Light, the records of Quria's power as well as how it would know what Savathûn plans to do. The waking simulation, the access to the Vex network, the knowledge of Hive words, the knowledge of the Worms and the use of identities as a potential coverup. That points to Quria.
The possibility that this could all be leading every Guardian into a trap where Savathûn could then just walk up to the Traveler and kill it, the use of Hive words, Worm-knowledge, the revelation of the inner workings of Savathûn's High Coven, the cunning nature and the second part of the entry "Thank You". That points to Savathûn.>