Nobody tells stories about technology, genetics, and ethics like Nancy Kress. Set in the near future, Steal Across the Sky features aliens who have landed on the Moon and have sent a message to Earth: we've been here before, and we did something bad to you, so send us some Witnesses and we'll reveal it to them. There are lots of volunteers to Witness for the aliens (who call themselves the Atoners) and a batch is selected. But these Witnesses seem to have nothing in common other than their age; none are over 30.

The Witnesses are divided into groups of three, given training, and sent off into the universe to land on sets of two planets. One planet holds the key to the other, and to the Atoners' crime. When the explosive secret is revealed to Earthlings, it throws the planet into crisis - the knowledge that something essential has been robbed from humanity has the power to change a fundamental understanding of ourselves, and our existence. That's when the story really takes off - now that Earth knows what has been done to its people, how will the Atoners atone? More importantly, should they?

This is a great pick for lovers of traditional scifi as well as those who enjoy thorny ethical issues. It's got some great, well-developed characters that you'll love or love to hate; they keep the tension and the plot moving briskly forward to the inevitable end.