I begin with the letter 'A', and to represent it, I choose Asimov, or rather Isaac Asimov to be specific. Writer; futurist; creator of the Three Laws of Robotics; and maker of many science fiction tales. Some of his most notable works include: the Foundation, Galactic Empire, Robot, and Lucky Starr series of books. As time marched on ever forward, he later united these seemingly separate series into one "future history", though Lucky Starr is not included so far as I can recall.
Considered one of the top three science fiction writers that dominated the time, of which the others were respectively, Arthur C. Clarke and Robert A. Heinlein, Asimov also wrote works of mystery and fantasy, along with a fair spattering of non-fiction.
While I almost equally enjoy the works of Arthur C. Clarke, and there are numerous other interests and entries suitable for my representation of 'A', Asimov remains simply for the impact his worlds have imprinted on myself.
Any science fiction nut worth his salt is familiar with Asimov's Three Laws of Robotics. It frequently turns up in various stories in all seriousness and in jest, but also presents a solid platform of which to oppose the non-sense that any and all A.I. will by nature, eventually turn upon its creators. Not only do the rules govern safer synthetic thought processes, it also presents a challenge to the individual's integrity, and whether or not the rules can be broken. The Robot book series has left its own mark on where my thoughts turn to, and how to approach artificial intelligence, and the potential that true, unshackled synthetic intelligence might possess and utilize. One of my favorite Asimov works is The Robots of Dawn, third in the series, and an interesting "whodunnit" adventure for plainclothesman, Lije Baley.
The man can write an intricate story and plot, laced with interesting and diverse characters, whether or not they're human. Some have criticized his style of prose, for its often direct or sparse way, but its fashion leaves room for the mind to interpret and develop. Another intriguing aspect of all but a select few of works feature worlds devoid of non-humanity, where even at times robots do not accompany Man. It is a neat principle that avoids what is known as 'Clement's Paradox', or the effects of time and longevity on sentient races, and the chances of encountering them. This is also handy for certain interpretations of a humanist Traveller universe.
Plus, Alien has been so oft used for the 'A' entry, I found it a little passe to follow suit.