According to what purports to be a memoir– “The Nightmare Brethren” –most aliens are not invading the Earth. This is illegal and as well as dangerous. However there are other types of alien/human transactions that can be hugely profitable. The Bureau of Alien Interactions is in charge of preventing the more destructive exchanges. These can involve advanced devices, precious metals, gems, or drugs. All of which are readily available from one or more of the alien worlds. To protect the Earth we have a ragtag crowd of aliens who officially do not exist. This defensive force is made possible by the Ur technology; non-physical mechanisms that are old beyond time, though slightly erratic. This last term also describes our crack team of human troubleshooters. Nobody understands the recruitment process.
The entire narrative is currently in the form of an ebook. Available on Earth among other planets. Caution: Live organisms are not acceptable as payment.