Source: Tasha's Cauldron of Everything
Wondrous item, uncommon (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 +1 bonus to spell attack rolls and to the saving throw DCs of your wizard spells.
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.