The Measure

One morning, everyone on Earth aged twenty-two and older wakes up to find a small wooden box on their doorstep. Same box, same inscription: "The measure of your life lies within." Inside is a single piece of string. No sender, no explanation, no instructions. You can open your box or leave it shut. Either way, everyone around you is facing the same choice, and the world starts rearranging itself around the answers.

353 pages. Goodreads 3.96 with 400k+ ratings.