1. Isaac Asimov
Since 2007 my favorite author, I collect the works of the dear doctor; an entire bookcase is devoted to him. This includes the full sweep of the meta-Foundation series, including the Robots and Empire novels. Besides this I have well over a dozen short story collections, several score of essay collections, and a multitude of proper nonfiction, mostly science with a little history sown in. Asimov's historical works are hard to find and always dearly priced. I must have a hundred or so of his works.
2. K.A. Applegate
Back in middle school I was a devotee of the Animorphs book series, and unlike the other books of my childhood, I've not yet lost these. I've given away a lot of the books to my niece and nephew, but still have 60-odd paperbacks in a wooden trunk.
3. John Grisham
I've read everything Grisham has read, and have owned all of it at one point. I've given a book or two away over the years, since his recent books are hit and miss, but most are still around.
4. Ann M. Martin
Although I've retained my Animorphs books largely by accident (I never think of them), I hold on to another series from middle school, California Diaries. It consists of six California teenagers, friends all, keeping journals about their lives. Each has different issues -- one has divorced parents, another is anorexic, that sort of thing. I probably wouldn't have gotten into the stories if not for the fifth book, the lone male.
5. J. K. Rowling
It helps that I have some of the books in German, too.
6. Frances and Joseph Gies
Years ago I read Life in a Medieval City and since then the Gies have been historical favorites; I read most of their works here back in 2007 and 2008, if memory serves, and unlike a lot of my material I actually purchased some of these after returning my library copies, just so I could read them again.
7. H.G. Wells
I own a series of Wells' work, but I haven't read most of them despite very good intentions.
8.Christopher L. Bennett
9. David Mack
I group Messrs Bennett and Mack together because they're my favorite Trek authors, and if I have a choice between a Trek book by either of them and another author, I'll invariably choose them. They're quite different, and Gene help me if I ever have to choose between the two.
10. Carl Sagan
I don't own nearly as much of Sagan as I've read, but Sagan's books hold the crown distinction: I read all of them first from the library, and then bought them.