Although published as three separate books, the Tenth Planet trilogy should really have been released as a single compilation. None of the three books are able to stand on their own. As a set, they are a light but engaging read. These books are pure escapist fun, with stereotypical characters, some plausible pseudo science and a fast paced plot. As long as you are not expecting great literature like Gene Wolfe and are prepared for sci fi that is more in the spirit of Edgar Rice Burroughs, you will enjoy this series.
In the second book, Earth has learned the terrible secret of the tenth planet. Not content merely to defend themselves against the rapacious Malmuria, Earth breaks out it's stock of antiquated nuclear weapons and prepares to take the battle to the Malmurians on their home turf.
If you enjoyed this series then you will probably like the Heritage Trilogy by Ian Douglas, which is far better written.