Shatner (or his ghostwriter) has crafted a story about an aging Captain Kirk (the time period falls after the last original-cast ST movie as the Federation and the Klingon Empire are beginning to move cautiously to detente) who grabs at what appears to be a chance for eternal youth on a planet that turns out to have a horrendous secret. While it's interesting to see a genre book that attempts to take the established character into new psychological / emotional territory, it's really difficult to believe Kirk wouldn't have immediately seen through the ruse at the base of the story.
A great read for Star Trek fans, especially if you like William Shatner's writing.