My two most active buy-on-sight authors are Lois McMaster Bujold and Diane Duane.
There is a Lois McMaster Bujold community on LJ:
lmbujold. There is also an official website (http://www.dendarii.com) and a mailing list associated with the website that Lois Herself has presence on. Flagged for the attention of new reader
naraht (noticing you, actually, was what prompted this entry, as I recognised the name): Diane Duane's blog has the RSS feed 
outofambit, and then there is the community youngwizards. Anne McCaffrey and Mercedes Lackey are no longer buy on sight authors, due to one or more of the following factors:
- decline in quality of work
- me growing up
John M. Ford would be a buy on sight author, except that I seem to own many books that he's written and haven't run into one of his that I don't have in years, and that I'm not seeing much new material from him.
Robin McKinley had been a buy on sight author, but had been inactive for a while; Patricia McKillip is becoming one. C.J. Cherryh has been one for a while, but her writing is developing more, for me, into read-at-library-first series, and buy-on-sight series.
I will buy other books, but I have to get to know them first: someone must recommend them to me, or I must have read them before, or I must be sold on them by cover art grabbing my eye, and read enough of it to know that I'll want to read the rest and re-read it.
Re-readability is the real test of whether or not I think I'll want to buy a book. Books are not disposable, to me. I read fast enough that I zoom through books in a few hours (if I have the time) to a few days (if I'm somewhat busy) or up to two weeks (if I'm absolutely jammed for time). If I buy a book, I'll want to keep it, and if I keep it, I'll either want it for reference later, or I'll want to read it more than once again.
