Back of Beyond, by C. J. Box

I really liked the first half of this book.  It was set up well with a cop trying to solve a murder which led him to track down a horse-back trip into Yellowstone that both the murderer and his son were on.  The third quarter set up lots of mysteries and odd behavior from the members on the trip; it turns out this was just too much to wrap up in a natural way.  In the last quarter, a lot of back story was brought in to explain all the odd behavior and it didn't flow naturally from what came before.  It's a good example of deus ex machina and for me it ruined what was a good book up to that point.  I enjoyed Box's previous Blue Heaven because it had all the elements from the first three-fourths of the book come together naturally in a satisfying resolution at the end.  Now I wonder if the success he had in Blue Heaven was more by accident than design.