Lucky Starr and the rings of Saturn, (A Signet book)
by Isaac Asimov
Category: Life on other planets

Six weeks after returning from the Jovian system, David "Lucky" Starr learns that Jack Dorrance, the chief of a Sirian spy ring uncovered by Starr in the Jovian system, has escaped from Earth. Starr tracks Dorrance down in the Saturnian system and discovers that the Sirians have established a base on Titan.