The titles, Gersen's antagonists, and a few details of their plots, in order of publications:

The Star King. The antagonist is Attel Malagate, an alien renegade masquerading as a human being. The bait Gersen uses to trap him is an undeveloped and fantastically beautiful planet whose location is known only to Gersen, and on which Malagate desires to become the father of a new race.
The Killing Machine - Kokkor Hekkus, a 'hormagaunt', has prolonged his life by the vivisection of human beings to obtain hormones and other substances from their living bodies. But eternal life can be boring, and so he has converted the lost planet Thamber into a 'stage' wherein he fulfills his fantasies by acting out various roles in scenarios of his own invention.
The Palace of Love - Viole Falushe, an impotent meglomaniac obsessed with sex. This novel contains some of Vance's most compelling and unforgetable characters, such as the mad poet, Navarth, who has a central role.
The Face - Lens Larque, a supercriminal sadist and trickster. In the course of the novel the protagonist and the villain come to have certain experiences in common leading to one of the most humorous endings in all Vance's work.
The Book of Dreams - Howard Allen Treesong, a 'chaoticist', who embodies elements of all the forgoing.