unsigned3_unknown.md, located at -112 / 6 / 311 is found in a large underground section in the overworld.
The book is located on the tilder server.
State of the mess
===================
Currently there's not much.
Come visit U-Town !
Build your house there !
Enjoy underwater vacation !
Relaxation (not) guaranted !
Read more in the book at U-town's spawn
I have no idea what to build in this embassy. Enjoy this hand-crafted part of the ocean.
If you want to know where it is, look at the cartographer's guild or ask the_pumpkin_man to tp you there