Source: Arcana Unleashed
Wondrous Item, Rare (Requires Attunement)
Hundreds of pages of illustrations, tables, and diagrams about a wide variety of monsters fill this magical tome.
Charm Monster. While holding the book, you can cast the Charm Monster spell (spell save DC 15) from it. The target has Disadvantage on the saving throw against this spell if it isn’t a Humanoid. You can’t cast this spell again in this way until the next dawn.
Extensive Knowledge. When you become attuned to the book, you gain proficiency in one of the following skills of your choice: Arcana, History, Nature, or Religion. Whenever you finish a Long Rest, you can change your choice. If your Attunement to the book ends, you lose this proficiency.
Monster Primer. While carrying the book, whenever you take a Study action to make an Intelligence (Arcana) check to recall information about an Aberration, Construct, Elemental, Fey, or Monstrosity you can see, on a successful check, you learn whether that creature has any Immunities, Resistances, or Vulnerabilities. If the creature has any, you know what they are.