I discovered Asimov's work at an early age – I think it may have been The Caves of Steelthat was the first thing of his that I read – and resolved to read everything of his that I could lay my hands on. Little did I know what a daunting task that would be – there was a time while I was in college that he was literally writing them faster than I could read them (I could have kept up easily if I read nothing else but his stuff, but the demands of college – and of other reading matter – intervened. Asimov wrote extensively on such a wide variety of subjects (over 200 titles), it seems restrictive to label him a science fiction author, but that's where he started and that's what he always considered himself.
First and foremost, the Foundation trilogy: Foundation, Foundation and Empire, and Second Foundation. I have never read the later books that extended the trilogy and tied it in with the Robots books (Foundation's Edge, etc.), but I read these first three over and over. Asimov said he took his inspiration (at least in part) from the Roman Empire, which was an attraction to me, and his interstellar empire was galactic in scope, and makes the Third Imperium of Traveller seem rather small by comparison.
As part of his phenomenal output, Asimov wrote several histories, including a book each on the Roman Republic and the Roman Empire. He did a two-volume biblical commentary, and a commentary on Shakespeare.
One of the things he is most famous for, however, are the "Three Laws of Robotics" outlined, explored, and examined in detail in his Robots books: I, Robot, The Caves of Steel, The Naked sun, et al.
Go to Amazon.com, do a search for "Isaac Asimov" and be amazed by the number of titles that turn up – I was.
