Lord Peter, by Dorothy Sayers (short story collection).
I'm not even sure how often I've read this collection of stories -- four, maybe five, times, maybe even more -- and I was a pretty late arrival to the Lord Peter party. I didn't discovery Dorothy Sayers until I was well into my 20s.
The mysteries are well thought out, often complicated, and usually intriguing. And that is not even the best part of the stories. Lord Peter is. Or rather, the brilliant writing bringing Lord Peter to life is.