Trick or Treat on Specter Street: Phantom Pages

Hey everyone! 👋

I've been working through Trick or Treat and having a blast so far. However, I've hit a wall on Question #3 of Phantom Pages and could use some help!

What I've tried:

• Found the library and answer to #2
• Examined all the book titles
• Identified 3 titles that have numbers at the end
• Reviewed the available hints

I have these books with the numbers (horror, swamp and mask) but I'm not sure how to combine or use anything to create the 9 digits authorization code.

Am I missing a pattern? Should I be looking at something else?

Any hints or help would be greatly appreciated!

Thanks in advance!