Champ: Y'know how they say teenagers have no fear of death? That they're never gonna get hurt? Well not me, OK? I was born with the gift of fear.
R.L. Stine: All the monsters I've ever created are locked inside these books. When they open.
Hallway Player: Hello, Mr. Stine.
R.L. Stine: Hello, Mr. Black.
Zach Cooper: She's locked in this house and her dad's a psychopath.
Champ: Does she have a friend?
Slappy: You have the right to remain... silent.
Chosen answer: He found the book in his house, saw the title, and just naturally assumed that they were there because the snowman needs to be in a place where it is cold.
The book was with the kids not in the house.
Well my guess is that he followed the destruction that the abominable snowman left behind. He also must have checked his books over as well and saw which one was missing because there were some books on the floor.