This book spawns in random locations throughout Duvencrune. There are youtube videos and imgur catalogs with the pictures of the 8 or so locations.

There is one per server, but any player who already found it can not see it; so your friends who found it cannot search servers for you; you must find it yourself.

Once you find it, spend 200 energy to 'read' it and start the questline:
https://bdocodex.com/de/quest/7111/1/