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]