The Enterprise is at the Galactic Core, charting black holes and searching for signs of Klingon activity, when they encounter a Federation shuttlecraft, identified as coming from the USS Rickover, lost more than twenty years prior.
The shuttlecraft is piloted by a madman calling himself Jesus Christ, who Kirk recognizes to be Thomas Clayton, Kirk's old roommate at the academy (he must have had a lot of those--in Star Trek Log Seven, we learn that he had a Klingon roommate named Kumara, who returns in Star Trek Log Ten).
Clayton has come out of a Dyson sphere which the Enterprise soon finds itself pulled into. They find within a place called Lyra (McCoy initially suggests Pellucidar as a name), apparently ruled over by a god called Ay-nab, who is the sun.
Nothing very interesting happens for a hundred pages or so, and Kirk telepathically convinces Ay-nab to send the Enterprise back out before the whole Dyson sphere falls into a black hole. Spock ends the story unsure of whether the most logical explanation is that Ay-nab is a god, a telepathic computer, or what. The end.
Although there's a little exposition about what Ay-nab is doing (punishing the Lyrans for repeatedly starting wars), it's not satisfying.
The only alien that gets any meaningful amount of 'screen time', Ola, doesn't do much other than wring her hands and follow Kirk around, until she saves them all with the power of love. When we meet her, Kirk's appraisal isn't terribly inspiring:
And half the book later, when Ola has risked her life to help Kirk find Uhura (who had been kidnapped):
Well, it's not just Kirk and Spock who don't seem to think much of women, either:
Even Uhura doesn't think of Chapel except in relation to Spock.
Enough nitpicking. The book's not very interesting. Dyson sphere, hollow world, whatever. It's been done. By Trek, even. Ditto the space god ruling over the primitive people who once had more advanced tech. It's like Trek's Greatest Hits Volume 3: More B-sides.