Manual of Quickness of Action
Wondrous Item, very rare · 2024 rules, SRD 5.2
This book contains coordination and balance exercises, and its words are charged with magic.If you spend 48 hours over a period of 6 days or fewer studying the book's contents and practicing its guidelines, your Dexterity increases by 2, to a maximum of 30. The manual then loses its magic but regains it in a century.
Also in the 2014 rules (SRD 5.1).