Ask 2 readers what makes a great mystery and you'll get 2 answers, 6 for 6, 10 for 10, and so on. But they'll probably agree that the sleuth, protagonist, hero, or whatever has to appeal to the reader. Maybe he or she is sympathetic, like Odd Thomas, or heroic like Jack Reacher, or irritating-but-determined like Columbo. Somehow we have to want him to succeed, and we have to feel at the end that…