In the Cube by David Alexander Smith (PB, 1994 MM Ed., *LN*)
Back cover description: "Future Boston A century from now, Boston is like no other place on Earth: an enormous Cube containing millions of human, cybernetic and alien beings. The city is slowly sinking into the sea, but it's also the site of Earth's only interstellar port- a gateway that has brought dozens of extraterrestrial races to the Cube. And as Private Investigator Beverly O'Meara and her alien sidekick Akktri are about to find out, humans aren't the only ones who can be greedy. Or resort to murder..."