Uncharted Stars by Andre Norton

Uncharted Stars is the somewhat uninspired sequel to The Zero Stone. Most of the exciting elements that were present in the first novel are missing here, and the only scene that contains a great deal of action is when Murdoc and Eet invade Waystar, the secret lair of the Guild built on a Forerunner space station in a dead sector of space. The rest of seems like Norton felt she had to finish the duo's story, but really wasn't all that excited about it.

Flush with the loot from their reward for turning over the zero stone ring to the Patrol, Murdoc and Eet have purchased a ship, and are trying to find a pilot. Somebody has blacklisted them, however, so they have to search the dregs of the spaceport to find a drug-addled delisted pilot, Rizk (risky?) to take them on the next stage of their treasure hunt. Murdoc's first trading venture doesn't go as well as he had hoped, when he is out-bargained by a more experienced trader, so he makes a quick change of plans to turn a profit after all.

But, when he returns to civilization with some beautiful and valuable greenstones, he finds out that his goods have been blacklisted as well, and he must once more turn to the seedy underside of the trade, using the knowledge of his father's Guild connections to find a less reputable buyer for his wares. While dispensing of the stones, Eet's telepathic powers alert them to a pirate raid that is about to take place on a Zacathan archaeological site on another world, so they rush off to try to warn the Zacathans, but arrive a bit too late for all except one survivor of the raid.

The thieves made off with several valuable Forerunner artifacts, including a bowl which is actually a star map containing clues to the location of the source of the zero stones and their unimaginable powers. Murdoc, Eet, Rizk and Zilwrich (the Zacathan) rush off in pursuit of the pirates and, quite improbably, are able to infiltrate Waystar and steal back the star map, so they can track down the zero stone home world.

The ending of the book contains a surprising twist, but the most surprising thing of all is that Eet and Murdoc never showed up again with further adventures. Norton must have decided to move on to newer things, though the interstellar cultures she uses as background here show up in many of her other novels.