Have you ever played a Mystic deck built around recurring Premonition every turn?

Book of Living Myths is just a persistent Premonition attached to an Asset for Kōhaku Narukami decks. Assuming you keep an eye on the number of Blessing / Curse tokens and have ways to manipulate the Blurse distribution in the chaos bag, Book of Living Myths can guarantee that you draw at least 1 symbol and at least 1 Curse / Blessing token of your choice per turn. That has some unique deckbuilding opportunities:

For cards that only care about "Symbols" being revealed (assuming you use the Taboo list), like Jewel of Aureolus, Read the Signs, Hypnotic Gaze, or Ritual Candles, then Book of Living Myths guarantees that you always satisfy the symbol conditions.

For cards that actively require a Blessing or Curse token to be drawn, like Eye of Chaos, Ofuda, Beloved, or Fey, then Book of Living Myths can help Kohaku reveal Blessings / Curses consistently where other Blurse Mystics would need to play Favor of the Sun / Favor of the Moon + Olive McBride to match him. Most Blessing and Curse cards are balanced around the fact that you're unlikely to consistently reveal them from the token bag when you need them; assuming you build your deck to take advantage of both Blessings AND Curses, Book of Living Myths removes that random chance altogether to the point where it almost feels like Kohaku is cheating.