A unique discovery. Books of this kind grew out of university culture in Germany in the 16th century, but by the 17th century had become a form of social networking used by people of all professions and stages in life---the 17th century Facebook. Through these inscriptions, they built relationships, documented their worldliness, and created an image of who they were through the people they had gathered.
Useful links:
https://publicdomainreview.org/essay/boys-will-be-boys-playing-around-in-a-17th-century-friendship-book
https://www.smithsonianmag.com/smart-news/german-library-acquires-friendship-book-almost-400-years-after-trying-buy-it-first-time-180975693
