Originators (Netherspace #2) by Andrew Lane

Contact with the alien race, the Gliese, was made forty years ago, but communication with them remains impossible. There is trade, but on seemingly inexplicable terms; anti-gravity technology traded for a bicycle tyre. Human science has become fixated on understanding the alien technology, but with little success. The world may be a better place but it's no longer our own, with mankind dependent on inexplicable Faster-Than-Light technology, controlled by the aliens.

As humanity begins to colonize the stars, we are still dependent on mysterious aliens we do not understand. It falls upon the unlikely team of a conceptual artist, Marc Keislack, and assassin, Kara Jones, to embark on a dangerous mission that will unearth the mystery of the Gliese.