First of all, Purgatory's Key is the third and final book of the Trek 50th-anniversary trilogy Legacies. The first two books delivered exciting action and an intriguing mystery concerning the Transfer Key and the menacing Jatohr. Ward & Dilmore's entry does a good job in some respects and a poor job in others. First the good: it capitalizes on the Jatohr element. They are less the two-dimensional villains of the first two books and are revealed to be a rather interesting alien species... but there could have been more. The strange dimension that they come from never did make any sense, and the authors did nothing to explain how such a species could come from such an ethereal place. What we're left with is a gaping plot hole... too bad, because this one had potential!
Another good thing is the Klingon drama. This subplot draws on the strengths of the "Shakespearean" TNG Klingon stories, and it even drops hints as to how Gorkon comes to be a major player by the time The Undiscovered Country rolls around.
Good things too: Sarek and Joanna McCoy get a few good character moments, while Captain Una holds her own. I feel that this trilogy overall has been a good Una story, leaving me wanting more, whether those stories be in the April, Pike, or Kirk era.
Bad things: the aforementioned Jatohr failure. Also, the trilogy hyped the Transfer Key as a secret passed from April to Pike to Kirk... and yet, Pike makes no appearance, just a paltry piece of lip service from Una. The Transfer Key is an interesting plot device, and I would have liked to see its potential realized to a greater degree.
Moreover: the climactic emergency in the Jatohr citadel, while in concept a worthy one, is never developed to the heights that the exciting climaxes of the previous two novels reach. And the last three chapters are pretty boring epilogues. A better-developed climax with a short epilogue chapter would have been preferable.
This review probably sounds a bit harsh. I did like the book most of the way, despite lamenting Pike's absence. The action is well described, the characters are almost always convincingly "voiced", and things move along at an enjoyable clip. I just felt that the potential of this book was not reached.