Stephen King once compared his work to the literary equivalent of a Big Mac and fries. Quick, easy, maybe unhealthy, and might make you feel guilty for having consumed it. I feel that this is the perfect metaphor for Finders Keepers.
I read this in a matter of hours. I'll admit, I did a little speed reading and skimmed some parts. Sometimes to the point that I wondered if this would have made a better Stephen King novella than novel. If you want a better book about fan obsession, read Misery. These "Bill Hodges" books are King's jaunt into hard boiled detective realism.
I felt like Hodges is an alter ego of King since Mr. Mercedes. He's a restless, aging baby boomer, who is looking for a satisfying final act.
As for the content of the book, I would compare it to a pulp novel. Murder, crime, people in peril. Nothing really new. Sadly for King to experiment with this genre, he doesn't bring much new to it. However, I've read a few mysteries and thrillers in the past year that relied on the old cliche of a villain undone by a secret recording. One of those written by JK Rowling under her pseudonym. At least King didn't resort to that old trope.
I hope Stephen King goes back to horror soon. I also agree that the element of the supernatural at the end seems to go away from the realism King was trying to establish with this series.