Certainly mystery authors change over time - most of us do that. They seem to change their style and context, too. For instance, when Lilian Jackson Braun's Cat Who... series begins, the sleuth, Jim Qwilleran, is an all-but-down-and-out reporter for a big-city newspaper. The characters he encounters are city people, by and large, and he lives the "city life." He dates several women, and smokes a pipe. As the series moves on, Qwilleran moves to Pickax, located in Moose County ("400 miles north of nowhere"). The setting of the series changes to a much more rural context, and Qwilleran changes, too. He inherits a vast fortune, gives up his pipe, and in other ways, seems to settle down. He dates just one person, Polly Duncan, throughout most of the later novels. Some people argue that Braun's focus has changed over time from the mystery itself to the characters that populate Pickax, and the relationships among them. In fact, I read one quite scathing review of The Cat Who Had 60 Whiskers that mentioned that there was hardly even a murder in the novel, although in earlier novels, the murder and Qwilleran's investigation of it had been central to the plots. Whether or not that's true, there's no denying that the series changed over time.
I think Tony Hillerman's novels changed over time, too. In his very early Joe Leaphorn novels (The Blessing Way is one example), Leaphorn seemed much less reflective and much more a man of action. As the novels went on, and Jim Chee was introduced as another major character, the characters grew and changed (natural enough, as people do change as they get older). In later novels, there was less action (although certainly never a dull moment!) and more thinking and reflecting. I think Hillerman did a truly effective job of letting his sleuths age and change over time, and approach solving mysteries in different ways. In Hillerman's case, the series did get better over time, to me.
Laurien Berenson's Melanie Travis has changed over time, too. As the series moves on, she remarries, raises her son, learns much about showing Standard Poodles, and gains more confidence (and a reputation as a sleuth). To me, that's part of the series' appeal: we get to see the charaacters as they grow and develop.
I would even argue that Agatha Christie's style changed over time. Recently, I re-read The Mysterious Affair at Styles, in which Hercule Poirot is introduced. It's quite different in style and in the kind of murder that Poirot investigates from Christie's later mysteries (e.g. Hickory, Dickory, Death and The Clocks). The dialogue is even different. It seems to me that the dialogue in Christie's later novels seemed more natural, almost more vernacular. Agatha Christie's characters developed over time, too, of course, and that's one of the many things that has made me such a fan.
I hope that my style and characters get better over time as I continue my Joel Williams series. I think that's what an author should strive to do. I'd like to think that we writers learn from the first novels in our series, and hone our characters, style, and so on. I know that the more time I spend thinking and writing about Joel Williams, the better I know him and the people in his life, and therefore, the better I can write about him.
What do you think? Do you think mystery series get better over time? Do you think they get stale?