No, I don't think you are fooling yourself-some series fade out, others strengthen-I imagine that sometimes it gets boring for the authors as well, always dealing with the same characters-but if, for instance, a little old lady who sketches and wears ugg boots is what you started with, and what your readers have embraced and bought, then you are stuck with her!
I agree with you about Elizabeth Peters-when she stopped doing broad satire in the vein of the Victorian adventure novels, her books were stronger. I would love another one.

I think PD James stayed strong through her Dalgleish series, although I know some people don't like the tidy ending of the last one. Martha Grimes was weakening for a while, but I haven't read Dust.
I just finished reading 30some books in a series written by Sara Woods, and while some of the "intro the characters for new readers" did get a bit samey, I didn't tire of the characters or the writing. Same with Elliot Roosevelt's series about Eleanor-were they great literature? Of course not. But I enjoyed the last one just as much as the first.
I am still enjoying Mary Daheim's series set in Alpine, Washington-and she is up to the 21st or so...but I rapidly tired of her other series-the Bed and Breakfast one.

Chris, the latest Stephanie Plum was disappointing. Of course I love the characters and Evanovich has been very successful with her formula, but as one of the reviewers over on Bibliophil stated, nothing ever changes: Stephanie is still lousy at her job, the Morelli or Ranger scenario is never resolved, Grandma is still going to viewings, there's no character growth and that may be what's wrong with this latest book. They're all doing the same thing over and over again - it's time for something new to happen.

Cece, I have at least ten of the Amelia Peabody books in my TBR pile. I loved the first two, but just haven't gotten back to them. I'm glad to hear the series holds up.

I think it's only natural that series should change over time. Some become better and unfortunately some don't. For example, by now the regular characters in Lilian Jackson Braun's Cat who... books have become like dear old friends to me, but I have to admit that the last 3 or 4 books I read in the series were not exactly award material in the mystery department. I am now somewhere in the middle of J.D. Robb's In Death series, and the books are getting stronger, but they may well start to decline at some point.

It's not just mysteries series I think this can happen to all types.

Deborah, of course it applies to other genres as well. For example, if you took the first and latest of Terry Pratchett's Discworld books and compared them, you would hardly think they were written by the same person. I think they have got better over time, but some fans have complained that the humour has become darker and the newer books are therefore not as fun to read as the older ones.

Another example, IMHO, would be the Mitford series, by Jan Karon. At first, these seemed so fresh and charming, but over time, I have grown sick and tired of the characters and style.

An author needs to develop and grow, and a series almost repels that process-because the author must write to a formula of character and situation or risk disappointing readers-poor Sir Auther Conan Doyle is a famous example-he wasn't allowed to move on from Holmes and he grew to loathe the character. It is a Catch-22 for writers who depend on writing to support themselves and families-write what will sell is good business practice, if not always good writing practice. Sometimes a series that should have ended continues only because there is a market clamoring for another one, please!

Or, a series that should have ended continues only because the "A is for XXX" etc author is contracted to write through the alphabet!

(I picked up Grafton's "T" book on the new-books shelf at the library--twice, different days--and both times the premise sounded so weird to me that I put it back.)

that would be Arthur Conan Doyle.
I agree-I think Grafton ran out of steam back around M or N.

I have had to put the latest Plum down for a break, I was laughing so much! I do agree about the Grafton R and S books being weak but as she has returned to her original investigative PI concept, found T a lot better.
I think it is being unrealistic to expect to enjoy every book in a series and I go along with the thought that it is better to have one than not. I am sorry when an author dies and the series with them. Although in some cases another author continues and does it very well. I'm thinking of Tom Holt and the 'Lucia' books for example.
I did not, however, like 'Rhett Butler's People'.