It sure has been a long time since I read a mystery novel I enjoyed this much. And one I finished in little more than 12 hours, too!
It's not just the premise - that an asteroid will destroy the earth in 6 months' time - it's the stellar plotting and writing. I mean, murder mysteries are all of a kind. You have to have some action, and between that action you need to have interviews. Lots of interviews, with lots of characters, hopefully designed to advance that aforementioned action and hopefully also obscure who the killer is so the reader finishes the dang book.
Therefore, the utter absurdity of someone pursuing a murder case when the Earth will be destroyed real soon now is the first unique card played by the author. The second card is absolutely believable characters. Not just our hero but every single person placed in his path - witnesses to fellow detectives to the old lady at the phone booth - I really do mean every person is realized and feels completely real.
The third and final card is interviews that are not boring. Please, those of you who write mysteries, read this one, if only as an exercise. Notice how he does not follow tried and trite methods of questioning that include "where were you at 10pm last night?" with each and every bloody person. Notice how he gives the detective a few charming tics in those interviews, which will be darned important later on.
There's really a fourth card - the author has thought about he would feel if the world is ending, and he's thought about how others will feel. How society will act. What will be repugnant and what will be understandable. If you don't like mysteries, you could read it for this reason and get plenty out of it.