Tome of Leadership and Influence

This magical book details techniques for how to best influence others. Spending 48 hours over 6 days (or fewer) reading and practicing the book’s techniques increases your Charisma score and your maximum Charisma score by 2\. The tome then becomes a mundane item for a century before it regains its magic.

Source: Level Up Advanced 5e · OGL-1.0a · 5.1