Source: Tasha's Cauldron of Everything

Wondrous item, uncommon (+1), rare (+2), very rare (+3) (requires attunement by a wizard)

While you are holding this leather-bound book, you can use it as a spellcasting focus for your wizard spells, and you gain a bonus to spell attack rolls and to the saving throw DCs of your wizard spells. The bonus is determined by the book's rarity.

You can use this book as a spellbook. In addition, when you use your Arcane Recovery feature, you can increase the number of spell slot levels you regain by 1.