Final answer:
Carol discovers that Mr. Linden's book contains potentially dangerous and powerful knowledge, with Mr. Linden warning her of its ability to impact one's senses adversely.
Step-by-step explanation:
The startling discovery that Carol makes about Mr. Linden's book is that it possesses seemingly dangerous and powerful knowledge. The warning Mr. Linden gives her is encapsulated in the phrase, "It is dangerous to read in a sorcerer's books," acknowledging that there is material within the book that can upset one's mental balance or have detrimental effects.
In explaining the peril of the contents, he admits to being barely able to glance over certain pages without losing his senses, hence advising Carol to take heed. This revelation and warning suggest a mystical or profound nature to the book, hinting at consequences beyond mere intellectual understanding.