Most science fiction concerns alternative futures, but a particular branch poses "what-if" questions on the outcomes of history. The classic scenario is "what if the Germans won the Second World War", but there are many more subtle possibilities. In computer form, this approach is more common in adventure games and even there no great examples spring to mind. On the printed page it's a different matter.

Take The Alteration by Kingsley Amis. Although a mainstream writer, Amis has a long association with science fiction, and his book portrays modern day England without the reformation. The Catholic Church still holds sway, and the best choirboys are kept for the church through castration - the alteration of the title. Amis's book is a lovely example, beaten only by the master of this style, Keith Roberts.

Roberts' first attempt, Pavane, is brilliant. Again set in a world under church control, Pavane has a wonderful mix of modern people, a culture kept back to feudalism, and technology limited to the steam age. Roberts has gone on to produce other equally impressive strange futures in Kiteworld and Molly Zero. Great stuff. Future and past mingle in that other great SF theme,

time travel. Once again, H. G. Wells is there to the fore with The Time Machine. This isn't as readable as either of my two other recommendations, but has to be mentioned as setting the format for everyone to follow. Although time travel is such a strong concept, and generates many possibilities for paradoxes and confusion, it has not inspired any masterpieces in either the game or written world.