Same here - found I couldn't put the book down, but the ending was kind of "is that it?". Felt like there should have been more. Considering the connection to the Bill Hodges trilogy the book felt like they were setting up Ralph as Bill's successor.
King sometimes struggles to find suitable endings, imo (not always though), The Outsider is one of those. I think he knows this, I watched It Chapter Two at the weekend and all way through the film there's reference to characters not liking the ending of Bill Denborough's books, even with a cameo from King himself saying he didn't like the ending of a particular book. I think this may be a tongue in cheek reference to where he fails to find a "strong" ending to some of his books.
Enjoying this show though - but I don't see how they can pad this out for 10 episodes, it feels like the first two episodes have covered the first third to half of the novel already?