Tome of Lost Knowledge (Patreon)

Wondrous item, legendary (requires attunement by a wizard)

This gold-trimmed book's cover is embossed with a portrait of Ogden Fellweaver, a legendary wizard infamous for his ill-gotten knowledge. If you can attune to this item, you can use it as a spellbook and an arcane focus. If your attunement to the book ends, anything written in the book disappears.

Absorb Knowledge. Immediately after a creature that you can see within 60 feet of you casts a spell of 1st level or higher, you can use your reaction to make an Intelligence (Arcana) check. The DC equals 10 + the spell's level. On a success, the spell is magically added to the book in your own notation. You must be holding the book to use this property. If the added spell is a wizard spell and of a spell level you can prepare, you can prepare this spell just like your other spells. If the spell is not a wizard spell, its notation is garbled and impossible to use. Once this property has been used, it can't be used again until a number of days equal to the spell's level have passed.

Shift Knowledge. While using this book as your spellbook, you can replace up to 3 spells from your prepared list of spells when you finish a short rest. Once this property has been used, it can't be used again until the next dawn.

Unknown Knowledge. While holding the book, you can use an action to channel its hidden knowledge. When you do, you gain proficiency in one skill or tool of your choice until the next dawn. If you're already proficient in the chosen skill or tool, your proficiency bonus is doubled, instead. Once this property has been used, it can't be used again until the next dawn.

Curse. This book is cursed, and becoming attuned to it extends the curse to you. As long as you remain cursed, you are unwilling to part with the book, keeping it within reach at all times. In addition, you are compelled to use this book as your spellbook. Each time you prepare a spell from another spellbook, you take psychic damage equal to the spell's level.

There's so much you don't know you don't know, you know?