Trail of Cthulhu Rules Clarifications

Stability

Q: Do you have to make a Stability test when you read a tome?

A: No. [Source]

Q: So, it's when you use the Cthulhu Mythos information you gained from the tome that you become scared?

A: Pretty much, keeping in mind that if your Sanity was already at 10 minus your Cthulhu Mythos rating, your Sanity is lowered merely from adding the Cthulhu Mythos knowledge you just read. [Source]

Q: Can you spend negative points of Stability, if you're already in negative figures?

A: Down to -5, you definitely can. Beyond that, you are blasted, and are not permitted to do anything other than flee, do nothing, or make attacks on perceived dangers, so, I'd say no spends on anything else, although it's not explicit in the rules. (I wouldn't permit anyone to make a spend which takes them below -11, although that is not an official rule.) [Source]