From GameWatcher: “The Sinking City has got a surprisingly large number of Side Case quests, such as the Mystic Tomes that are strewn about the game world. The aforementioned one, in particular, is quite complex, so a short spoiler-less guide may be handy for wrapping it up.
As part of this guide, you will find information on how to get the Mystic Tomes side-quest, and what sort of loops this particular Side Case might send you on. We’ll try and keep it spoiler-free, though do consider yourself warned.”
Source: N4G PC The Sinking City Mystic Tomes Guide