This book contains all the knowledge needed to properly apply a skill, carefully gathered and written down by someone with a true grasp of the subject. The words within the treatise is charged with magic, which helps the reader understand and use the knowledge within. If you spend 40 hours (or one week of downtime) studying the text, you gain proficiency in the skill that the book is about. Once you do this, the book loses its magic, but regains it in 10 years.
Master's Treatise
Wondrous Item (book), rare (requires attunement)