From PowerPYX: “The Rarest Book is an item in Tales of Arise thats needed to complete Sub-Quest Bibliophile.
The Rarest Book is actually the Heavenly Gaze craftable weapon for Rinwell. She uses books as her weapon, because shes a mage and casts spells. You can craft it at any merchant in the game, who are found at any campfire.”
Source: N4G PC Tales of Arise How to Get the Rarest Book