Don't ask me--read Angela James' advice over at Carina Press. Query letters are your first contact with editors/agents. I guess getting them right is like putting on deoderant before you go to a job interview.
I cringed when I read Angela's first do-not: Don't start with a rhetorical question.
Buuu-uuut, I wail. I love rhetorical questions. Apparently, editors don't.
Wisdom: Give editors/agents what they want, not what you want them to want.