There is a moment in most good mysteries when the reader thinks, "There's no way they're getting out of this one!" It's a great, great thing for an author to create...if she can deliver an ending to match.
The book I'm reading has built to that moment, but as I read this morning's chapter I began to doubt that the characters will be able to recover to any believable degree. The bad guy will die, of course, in some suitably horrible way. But the world has been so disrupted, what with the nuclear strikes, international war, and otherworldly emanations, that it's pretty hard to imagine the restoration of any sort of satisfying serenity. Maybe for some readers that's okay. I think I'm going to object.
It doesn't have to be cataclysmic world events. There are books where the author creates a puzzle that seems unsolvable, a crisis that seems unsurmountable in the protag's life. I'm happiest when all things are explained in a satisfactory manner, but often there is so much emphasis on the tangled web of plot that nobody can unravel it all. In those cases the reader has to decide if the ride was enjoyable enough to justify the fact that we didn't end up where we should have. For me, when that stuff hits the fan, somebody had better get it all cleaned it up by the last page.