Netherspace (Netherspace #1) by Andrew Lane
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. 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?