I am wondering, on the wiki on the topic of Zoe's character it mentions a "mysterious book" sidequest. I have been in the cave north of amberguard several times, but have not seen such a book (it might be because I played on an older version of the game, 5.15 iirc). My question is how you obtain said book

Edit: downloaded the newer version, found "underground quarters" which had said book, so problem solved. Now another problem is that my characters I used for fighting (from the old version) all are unable to join my party (I assume because of high stress) and nothing will bring their stress down (not even supporter cheat code)

There are several sources of stress, but the one that usually frustrates people is slaves hating each other.  If you select a slave and click Statistics, then it will show all relationships with that slave on the right.  Group sex is the best way to resolve this.  Otherwise, unclean mansion, lack of food, sleeping in farm or jail, nudity rule, pregnancy, toxicity, high lust without sex, too many people sleeping in communal rooms, and strict headgirl are sources of stress.