Netherspace Series by Andrew Lane, Andy Lane, Nigel Foster

Aliens came to Earth forty years ago. Their anatomy proved unfathomable and all attempts at communication failed. But through trade, humanity gained technology that allowed them to colonise the stars. The price: live humans for every alien faster-than-light drive.Kara's sister was one of hundreds exchanged for this technology, and Kara has little love for aliens. So when she is drafted by GalDiv - the organisation that oversees alien trades - it is under duress... moreAliens came to Earth forty years ago. Their anatomy proved unfathomable and all attempts at communication failed. But through trade, humanity gained technology that allowed them to colonise the stars. The price: live humans for every alien faster-than-light drive.Kara's sister was one of hundreds exchanged for this technology, and Kara has little love for aliens. So when she is drafted by GalDiv - the organisation that oversees alien trades - it is under duress. A group of colonists have been kidnapped by aliens and taken to an uncharted planet, and an unusual team is to be sent to negotiate. As an ex-army sniper, Kara's role is clear. But artist Marc has no combat experience, although the team's pre-cog Tse is adamant that he has a part to play. All three know that success is unlikely. For how will they negotiate with aliens when communication between the species is impossible? less

Contact with an alien race, the Gliese, has been made, but communication is impossible. There is trade, but on seemingly inexplicable terms; anti-gravity technology was traded for a bicycle tyre. As we begin to colonize the stars, we're still dependant on the mysterious aliens who we still do not understand. It falls upon the unlikely team of a conceptual artist, Marc, and assassin, Kara, to embark on a mission that will unearth the mystery of the Gliese.