Wondrous Item, Very Rare
This book contains health and nutrition tips, 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 Constitution increases by 2, to a maximum of 30. The manual then loses its magic but regains it in a century.
Source: SRD 5.2.1 · CC-BY-4.0