Copycat's Spellbook

Wondrous item, uncommon (requires attunement)

The pages in this pocket-sized spellbook are blank, and nothing seems capable of marking them or removing them. If you're holding the book when you see a creature cast a spell, you can use your reaction to make an Intelligence (Arcana) check (DC 15 + the level of the spell). On a success, you learn the name and details of the spell being cast, which magically appear in the book if it has room. While notes on a spell are in the book, you can use them as a written source for the purposes of learning that spell (such as a wizard copying a new spell into their spellbook or a creature with the Ritual Caster feat or Book of Ancient Secrets eldritch invocation learning spells with the ritual tag).

The book has room for three spells. You can use your action while holding the book to erase one or more spells from it, freeing up room for future use, and all spells it contains are erased if your attunement to the copycat's spellbook ends.