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.
Mystic Tomes Guide
The Sinking City has received a pretty middling review score here at GameWatcher, but it is still a must for any fan of Lovecraftian media. It is a game of smarts, though, which is true to the developers' Sherlockian pedigree. Some cases aren't straightforward at all, and some might be necessary, as is the case with Mystic Tomes.
First things first, to start the Mystic Tomes Side Case, you will first need to have the Necronomicon DLC bought and installed. This DLC-exclusive side-quest can be kicked off from the protagonist's room in the Devil's Reef, where the Necronomicon will direct you to the room across the hall. There, you'll find 7 separate notes, each leading you to a specific book in Oakmont.
Most of it is somewhat straight-forward, with the game itself leading you from one location to the other. Of course, you should come prepared, as a fair few enemies will appear at some of these spots as well. Finding the books, however, shouldn't be too big of an issue, all things considered.
Your rewards for getting through the Mystic Tomes Side Case are a number of Knowledge Points, and the unique Cultist outfit for the protagonist, though fans of this setting will find the tomes themselves most intriguing. Happy hunting!