Kidnapped on Astarr by Joan Lowery Nixon, 1981.
This is part of the Kleep: Space Detective series.
Till’s mother, Falda, has mysteriously disappeared, and Till is sure that someone has abducted her. The only clue he has is an unfinished note that his mother left for him with the letters “RU” on it. He takes it to Kleep and her grandfather Arko, and the three of them puzzle over what it could mean. Arko and the kids decide that the two most likely things the letters could be part of are a kind of metal that Arko and Falda are using in the project they’re currently working on (“ruthenium”) or a group of people who are enemies of theirs, the Ruzenians. The people of Ruzena lived on Astarr before Kleep’s people arrived from Ruel (another possible “RU” word that they ruled out) and have resented their intrusion.
Arko decides that he will first visit the mine where they get their metal, hoping that Falda has gone there in connection with their project to create a new way to anchor small space ships at outer space docks. However, Kleep and Till can’t help but think that the Ruzenians have kidnapped Falda. Arko wants them to stay at the house with the robot Zibbit until he returns, but they feel like they can’t wait and decide to take Zibbit with them to investigate the Ruzenians.
It’s a harrowing journey through Ruzenian territory, through a dark forest with giant worms and singing trees whose music threatens to overtake their minds, but they do discover that is where Falda is being held prisoner. Unfortunately, Kleep, Till, and Zibbit are also captured. With the king of Ruzena suspicious of the projects that Arko and Falda are working on (he thinks they’re designing weapons, but they’re not), what can they do to escape or get help?