I remember reading this book when I was about 12 or 13, so it's a middle-school reading level and published around 2005 or earlier.
It's about a group of kids living in a wooded Minnesotan town who, after some set of events, find their world has magic monsters in it.
I tagged it as horror because I remember the book having darker tones when I was a kid. I'm also pretty sure the conventional fantasy creatures were a bit more twisted than their Harry Potter counterparts, and they were out to seriously hurt the kids in this book.
There also is/was an IRL insurance company named after the city in which the story is set.