Revelation (Netherspace #3)

It's half-way through the twenty-first century.
Contact with alien races was made forty years ago, but communication turns out to be impossible. We don't share a way of thinking or common sensory inputs with the aliens, let alone a grammar. But there is trade, done on a basis of putting things on a table and taking them off again until agreement is reached. There is no obvious pattern to the trades. Alien anti-gravity technology was traded for a bicycle tyre. Human science has become fixated on understanding alien technology - with little success. We can learn what it does and how to operate it. We don't know how it works - or how to fix it. The world may be a better place but it's no longer our own. We may be colonizing the stars, but we're dependant on inexplicable alien Faster-Than-Light technology. This is controlled by aliens we call the Gliese and is the only constant trade: FTL engines for human beings, any age or condition, as long as they're alive. We don't know what happens to them, but rumour says they are taken to a Nirvana where all illnesses will be healed. Their families are also very well compensated. Interstellar immigration and trade are central to the world's economy. There is no shortage of volunteers.