Today is the centenary of the birth of Isaac Asimov. (Actually, his exact birthdate is unknown, due to lack of records and calendric confusion in post-revolutionary Russia, where he was born [prior to coming to the US at the age of 3], but he celebrated January 2, 1920, so be it.)
I celebrate Asimov because he was my gateway drug into science fiction. I liked a lot of his fiction and still do, but I was introduced to him via non-fiction.
My first Asimov book was Words on the Map, a collection of brief etymologies of place names. I liked words and I liked maps, so it was an obvious choice. This book was published when I was 5 years old, but I would already have been capable of reading it then, and I suspect it was not much later that it was given me as a present. Certainly I got myself pegged as an Asimov reader, for it was within 3 or 4 years that I was being given collections of his science essays, from which I learned a great deal.
I didn't know they were from a science-fiction magazine, and indeed didn't know much about Asimov other than the books until Opus 100 was published. This sally through his first 99 books and why and how he came to write them came out when I was 12, and by this time I was identified enough as an Asimov reader that a copy instantly appeared in my hands as a gift.
So it had something to say about his SF, and included a few full short stories, including "The Last Question" and "The Feeling of Power," both of which instantly struck me and which I've never forgotten. But I did not go on and explore more of his SF, until one day about four years later when I noticed paperbacks of the Foundation trilogy sitting on the free loan shelf in my school library. Oh, yes, I remember hearing about these, so I might as well try them.
I swallowed them whole in a couple of days, and that's what made me a serious Asimov fiction reader.
I liked the way his writing, at its best, combined a coolness and clarity of presentation with an underlying passionate intensity of commitment to the material that was nevertheless always kept under control. I liked equally his combination of confidence in scientific fact with human-scale values. Unlike many other writers, he never let his passion for science and logical thinking carry him off into cold, inhuman realms.
When I found my way into SF fandom a couple more years later, I came in familiar with some works by four authors: Asimov and Clarke, both of whom I liked, and Heinlein and Bradbury, whom I didn't. That pointed me in the direction that my reading took. Of course, some of Asimov's later works were disappointing, but his fiction of the 1950s and his nonfiction of the 1960s are still the highlights of his achievements as a writer.
I had more to say about other aspects of Asimov's opinions and personality here.