Tome of Clear Thought

Source: D&D 5.2 SRD Magic Items

Wondrous Item, Very Rare

This book contains memory and logic 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 Intelligence increases by 2, to a maximum of 30. The manual then loses its magic but regains it in a century.