The knowledge the author refers to is the reader's awareness of common tropes and conventions associated with the horror genre. This includes an understanding that characters often face dire consequences when they act recklessly or venture into dangerous or forbidden places, such as attics, basements, or abandoned houses. Readers are conditioned to expect that these actions will lead to encounters with malevolent forces or threats.
This knowledge increases suspense because it sets up a psychological conflict between the character's actions and the reader's expectations. When a character enters a potentially dangerous location, the reader is not only anxious about what might happen next but also aware that bad outcomes are likely. This anticipation of danger, combined with the thrill of uncertainty about the specifics of the horror that may unfold, heightens the overall tension of the story. In the case of "Salem's Lot," the reader knows that Susan's curiosity and disregard of the inherent danger in approaching the source of evil will likely lead to her demise, making the moment she ventures into the basement all the more foreboding and suspenseful.