The Outsider is listed on Goodreads as the fourth book in the Finders Keepers series. I’m not sure that’s accurate. While one of the characters from that series crosses over into this book, that character doesn’t come on stage until halfway through the book. While they have a major influence on how the plot plays out, they are not the main character of the book.
Basically, if you love reading Stephen King, you’ll probably love this book. It is a blend of horror, mystery, and police procedural genres. I gave The Outsider 5 stars on Goodreads. It was a satisfying read.