Question: When Zach wrote that all of the monsters got pulled into the manuscript, why did Hannah get pulled in too? She wasn't a monster but a friendly ghost.
Answer: When they use the term "monsters", it isn't specially about evil creatures, it's anything R.L. Stine created in his books. All of Stine's creations were pulled into the book, including Hannah, who was never a real person. A ghost would be considered a monster, no matter how friendly or benevolent.
Question: While arguing with her father, Hannah screams in terror and Zach thinks it's a domestic disturbance. Why did she scream anyway?
Answer: She was feeling frustrated about not being able to live a normal life. Outbursts are not uncommon from people who have been bothered by the same problem for a while.
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.