"I am thinking about the buying habits of readers, and trying to subdivide them into several categories, in relation to a single author's works:
* Character fan. Follows the books about one character/group of characters, may not read other books by same author.
* Setting fan. Follows the books in a particular setting. May not read other books by same author.
* Story arc fan: Reader will follow the story arc through a trilogy or series, but may not read more books in a new story arc about the same characters/setting.
* Genre fan. Reads author's works in a particular genre, but not in other genres.
* Theme/review based. Readers who pick up the author's works based on theme, or reviews. This group has tastes with regards to the author that are harder to slot into the usual groups, and the author's works sometimes suit those tastes and sometimes don't.
* Former fan: Reader followed one of the patterns above, but for one reason or another stopped. "Former fans" aren't necessarily people who stopped reading an author entirely. They might be people who bought the first five books of a ten+ book series, and then lost interest in that series but are still interested in one or more other works by the same author.
* Author fan. Reads everything the author writes, regardless.
* Casual. Picked up one book, may or may not ever get another by that author.
Leaving aside authors who have departed this vale of tears (including, most recently and regrettably, joelrosenberg) , because I don't want to be up all night typing, and authors whose careers seem to have vanished thanks to the major publishers' exterminating their midlist authors, it would be easier for me to do this inside out by listing the handful of authors who I am still a rabid fan of: Michael Flynn and Michael Z. Williamson. I will read pretty much everything they publish. Marko Kloos, maybe, though he's a marginal case since pretty much everything he's written is either a sequel to Terms of Enlistment or a side story to those sequels. Jerry Pournelle. Bill James. Tim Powers.
I can't think of any authors for whom I'm a character fan; perhaps Jim Butcher for his Dresden Files series.
Setting fan: Brian McClellan's "Powder Mage" novels, James Daniel Ross' "Radiation Angels" stories, Chris Nuttall's "Ark Royal" books, and Tom Kratman's Carrera series. Ross might fall into the category of genre fan as well since he has some fantasy out that I haven't looked at. S.M. Stirling...looking strictly at his solo work, I like his Emberverse novels, but didn't really care for the Draka or the Nantucket novels.
Genre fan: haikujaguar's Spots The Space Marine and Letters Home are, with the exception of The Three Jaguars, the only stories of hers I've been interested in. Okay, A Rosary of Stones and Thorns. David Drake also falls in here: I have read all of his SF and horror, but so far haven't cracked any of his fantasy novels. C.J. Cherryh, likewise.
I'm not a theme/review fan. The last person whose reviews I trusted was P. Schuyler Miller, who used to do the book review column for Analog back in the days when I was still reading Analog. Glenn Reynolds turned me on to some good authors a few years ago but he seems to have gotten away from doing even the very brief reviews he used to do on Instapundit.
Former fan: Harry Turtledove. I used to be a big fan, but towards the end of the "American Empire" series it became evident that he was just phoning it in, and what used to be a merely annoying bigotry against Southerners got completely out of hand in those books as well. I finished the series, was sorry I'd wasted my time doing so, and haven't read a single book of his since. David Weber has also fallen into this category. The Honorverse has completely gotten out of hand, the Safehold novels didn't hold my interest, and at this point the only thing I want to see from him is a conclusion to the Prince Roger novels he's writing with John Ringo. Charles Stross is getting close to the line; the Merchant Princes novels seem to have ground to a halt now that they were getting really interesting (in the Chinese sense of the word) and the Laundry novels have become genuinely unpleasant. I have been warned about The Annihilation Score and am not sure I even want to read it.
Lois Bujold, oddly enough, falls into the "Casual" category. I read one Vorkosigan book, and it was okay, but it didn't make me want to run out and get another one. Neil Gaiman.