Mystery author P. D. James has attracted a large following over the years for her mysteries. While some of them (particularly the earlier ones) are traditional mysteries, her later books have tended towards the psychological-profile type of story. I'll admit I'm not a huge fan of the style, but it is certainly a major genre of modern mystery stories, and there are a great many people who do like the style.
So it is of considerable interest when an author such as James writes a non-fiction book of literary criticism about mysteries, to put forward some of her own views about where the mystery story has come from and where it may be headed. With that in mind, James's "Talking About Detective Fiction" is, I think, a very worthwhile read. She offers some definitions of the genre, looks back at both the Golden Age and classical tradition on the one hand and the hard-boiled school on the other, and gives an overview of what she sees to be the present state of the mystery story and where it is headed. Having just turned 90, with such a long and prolific career, her viewpoints are worth your consideration.
I have mixed feelings about the book - I think some of the choices she makes of authors to be discussed are a little odd, for example - but I do have to recommend it. It's a fairly quick read, well-written and quite lively. If you're serious about your mysteries and want to expand your horizons a bit, "Talking About Detective Fiction" can be a good starting point.