You have learned to cast Rituals to overcome supernatural threats.
Grimoire
Your Rituals are recorded in a grimoire, a Tiny object that weighs 3 pounds and contains 100 pages. You determine the grimoire's appearance and materials.
The grimoire starts with four level 1 Investigator spells of your choice. Detect Magic, Heroism, Memorize, and Transient Bulwark are recommended.
Whenever you gain an Investigator level after 1, you can add two Investigator spells of your choice to your grimoire. The Ritual Level column on the Investigator Feature table shows the maximum level of a spell you can add to your grimoire.
Ritual Casting
You can cast any spell as a Ritual if that spell has the Ritual tag and the spell is in your grimoire. You must read from the book to cast a spell in this way. You can't cast spells that are in your grimoire except as Rituals, unless you've learned them by other means.
Bonus Rituals
You can treat specific spells as if they have the Ritual tag, allowing you to add them to your grimoire and cast them as Rituals. These spells are marked in the Investigator Spells list.
Spellcasting Ability
Intelligence is your spellcasting ability for your Investigator spells.