This story is a sequel to the TOS episode "Shore Leave", in which the Enterprise happens upon a 'shore leave planet' that is designed just to satisfy, as Kirk noted, the need of complex minds for the simplicity of play.
The Enterprise has been overtaxed, lately (the stories in these novelizations are written as taking place in sequence), so Kirk asks for something special in the way of reward for the crew, and he gets it: approval for shore leave on the Shore Leave Planet, in the Omicron region.
Upon arriving, Uhura, Sulu, and McCoy beam down together and note that everything seems to be as it was when last they saw the planet, down to the appearance of Alice and the White Rabbit. They go their separate ways in order to enjoy their own–private–fantasies, but McCoy has scarcely come into view of the Southern mansion he dreamed up when he is set upon by armed playing cards, straight out of Alice, who attack him in deadly earnest. He manages to call for an emergency beam-up just in time to escape them.
Shore leave is canceled as the crew of the Enterprise strive to determine why the planet is attacking, why the Keeper didn't intervene, and what has happened to Uhura, who has vanished without a trace.
This story is pretty good, and translated well by Foster.
Mudd's Passion
Cutting shore leave somewhat short, the Enterprise is ordered to investigate the activity of an old 'friend', Harry Mudd, who we last saw in "I, Mudd". He is up to his old tricks, swindling people far and wide. This time, he's selling a love potion.
This story is very thin and no better for Foster's efforts.
The Magicks of Megas-Tu
The Enterprise is sent to investigate the unusual phenomena at the center of the galaxy, including a 'negative black hole' busily ejecting matter, which they presume to be the source of all matter in the galaxy, drawing its energy from a multitude of other universes. Then they begin to be drawn into a cone-shaped vortex which is drawing in–and destroying–matter, from which the Enterprise cannot escape. They gamble that it may be safer in the center of the vortex, and, passing through it, they find themselves in another place, strange to them, operating by no known laws.
The delicate equipment of the Enterprise does not take kindly to this lawlessness, and begins to fail. The crew, dependent on this equipment, begin to fail as well. When the situation has grown most desperate, the Enterprise is suddenly saved by a strange alien–half man, half goat–who appears on the bridge. He restores their environment with what appears to be magic, then introduces himself:
"Who am I? Oh, you want a name! Call me Baal." He paused thoughtfully. "Or Lucien. Yes, Lucien. But above all, call me friend." One finger fluttered skyward as he declaimed, "Never could I abandon those who have come so far to frolic with me . . . for such purpose you must have been sent."
Lucien introduces the to the planet Megas-Tu, where the physical laws correspond to what the humans would call magic. His people had ventured out of their own universe before and encountered Earth, but their welcome had not been so warm. When others of Lucien's people discover the humans, they quickly put them on trial for the crimes of their species, as exemplified by the Salem witch trials, in which, weakened by the distance from their own world, the Megans were persecuted and even burned.
Kirk argues that if humans were once so savage, they have changed, and continue to strive to change, to be better and more noble. The Megans accept that this may be so, but declare that Lucien still must be punished for bringing the humans to Megas-Tu. Kirk defends him, as well, accusing the Megans of being as cruel as they accused the humans of being. In so doing, he passes a secret test, proving by his concern for Lucien, known also as Lucifer, that humans truly have changed. Should humans again visit Megas-Tu, they would find a warmer welcome.
Where to begin with this one? The adaptation is good–superior to the original. It spends too long on the setup and not enough on the resolution, but it's still well done. As for the story, it was obvious to anyone just who a goat man named Lucien would turn out to be, but it was satisfying, all the same. Kirk and McCoy question whether Lucien was really the Lucifer of myth, and McCoy concedes that it doesn't really matter, except:
"It's just that--if he was, Jim--this would be the second time he was on the verge of being cast out. But thanks to you, this is the first time he was saved."
The author of this episode, Larry Brody, indicated that originally, the Enterprise was to meet God out in space, but that idea was nixed by the censors. But meeting the Devil in space was fine, and so the episode was born. This episode must have been influential, indeed. In the first episode of Star Trek: The Next Generation, "Encounter at Farpoint", Q puts the crew of the Enterprise on trial for the crimes of humanity, and Picard, too, argues that Q should consider whether humanity is presently as savage as in times past. Then in "Where No One Has Gone Before", the Enterprise is taken to the edge of the universe, and find it a strange place where reality is impacted by thought. Then, in Star Trek V: The Final Frontier, the Enterprise (under Kirk's command, this time) visits the center of the galaxy, where they find a godlike being who turns out to be evil.
In summary
The first and last stories in this are quite good, though the middle one is forgettable. That's a pretty good ratio for novelizations of television episodes. "Once Upon a Planet" is perfectly like any Trek episode you've ever seen, and "Mudd's Passion" is like most of the bad ones. "The Magicks of Megas-Tu" isn't a top-tier story, but it's pleasant enough, and interesting in how it presages later Trek. If you're a Trek fan looking for a little light reading, this book isn't bad.