This question came to me after reading the thread started about Angelina Jolie playing Patricia Cornwell's Kay Scarpetta in a future movie based on Cornwell's books.
A few that come to mind for me are:
Dean Koontz -- I loved his books when I was much younger, but after the 450th alien and/or government conspiracy story, I felt his novels were becoming a tad formulaic

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Stephen King-- He was always a little out there to me, but his older books, especially those under Richard Bachman were at least enjoyable.
Robin Cook -- I felt he too was becoming a little bogged down in formula.
Every now and then I will pick up a new release from one of these authors (or others I've liked in the past) and sadly it's like going back to an ex-boyfriend -- I quickly realize why we broke up in the first place!
SK was the first "real, grown up author" I read (IT, back in 6th grade!), so I have a soft spot for him. He flagged (no pun intended) for a while, and the last few books of the Gunslinger left something to be desired, but I will keep buying his books.
DK, on the other hand, I think found some sort of religion or spiritualism at the bottom of one of his Coronas because in the late '90s, I noticed a real change in his writing. He went from aliens and voodoo and DNA-shapechangers to weird stories that take place over a day and involve a dog (I know, he likes goldens) and the process of "finding yourself". That said, I will say that "From the Corner of His Eye", which I consider the turning point in DKs writing, is EXCELLENT, and recommend it to everyone.
King's work has certainly lost their luster. I can't think of the last King book I thought was truly quality story telling.
Koontz's work hasn't been great either...except for the Odd Thomas books! Those just seem to continue to improve.
Anne Rice's quality hasn't been the same since her health issues. I'll wonder forever just what might have been if not for the heart problems.
I'm a sucker for SKs Gunslinger Series, so that was probably the last one I loved from him. BUT, I have not read Duma Key or The Dome yet. I read the Cell, and I just thought it was dumb. I think I didn't 'get' it and I have to read it again....
I love the Odd Thomas books, too, but he delves again into way too much of the spiritualist, new-agey, hippy dippy trippy type stuff. Just tell me a story. Not 5 pages of how the sun makes you warm and happy. Ugh.
Michael Connolley has NOT deteriorated at all.
LOVE LOVE LOVE Connolley!! He can not get them out fast enough for me. I have every book (and I've noted on the insides of them what book they are in the Bosch Series).
And I will include John Saul - someone I really used to enjoy..
I used to read Saul, but he's just cheesy.
Koontz seems to have played out the guy's wife is held hostage plot a few times over, but I love his earlier work.
He could still do the hostage thing, maybe, if he gave the book some wheels and made it GO faster! Koontz' recent works seem to always go so slow. Instead of getting one paragraph on the look of someone's kitchen (The Good Guy, or whatever), we get like 2 pages worth of description. Move the plot forward already!
I agree with Dean Koontz. I loved those books in high school, now I can't stand them.
I like Middle of the Career DK. Some of his really early stuff was stupid (Demon Seed, anyone?), but by the 80s and 90s, he was putting out some good stuff (The House of Thunder, From the Corner of His Eye, Strangers) By the turn of the century, he was putting out drivel like The Taking, and Your Heart Belongs to Me. Blech.
Michael Connelly, Robert Crais, Jeffery Deaver and John Connolly are authors whose books I can't wait for...they all have great characters with great plots and excellent writing. To me their work keeps getting better and better.
Yay! Someone mentioned Deaver! I love him, too. The Lincoln Rhyme books are fabulous because it's like you are working the crime with him, rather than reading a story and get to the end where the Butler did it and you can't figure out HOW the butler did it.
Oh, I envy you a husband that reads the same books as you! It's like having a built in book club. My DH isn't a fiction reader, and none of my friends read the same kind of books I do, so I have no one to discuss my books with.
Yeah, this can suck! My hubs is a fantasy guy (David & Leigh Eddings, Dragonlance, etc) We can't talk about anything!
And I'll throw in my two cents here - give Iris Johansen a go, as well as Greg Illes. Both are excellent. Greg Iles can be a bit "intellectual", but it's great (for those of us who don't want Fluff Mysteries, but something with real bite, read Spandeau Phoenix - a story about WW2 Nazis.)