No, the language Stephen King uses is correct and natural, and understood easily by a native speaker. It would be understood easily even in speech. Stephen King is sometimes looked down on because he is so successful commercially, but I think he is a very skillful writer who knows how to use English well.
Suppose you talked to her before she found the food supplies. Suppose you asked "How will you feel if you find food?" She might have said "I would be very reassured." You ask "On a scale of 1 to 10, how reassured?" She says "9."
Even without being asked, she might have thought to herself "it would be very reassuring to find food." I say "she might have thought" because she might or might not have thought about it.
Now that she has found the food, she is somewhat reassured. If you ask her "On a scale of 1 to 10, how reassured are you," she says "3." The food supplies did not reassure her as much as she might have thought.