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 third and final book in this series, Earth and the Tenth Planet are locked in a battle for survival. Badly damaged and knowing that they are faced with extinction unless they can harvest most of Earth's resources, the Malmurians turn to their distant past and the spirits of their Elders to find weapons to subdue Earth. Meanwhile, the elite of Earth's scientists work frantically to develop new weapons in their fight against the alien menace.
If you enjoyed this series then you will probably like the Heritage Trilogy by Ian Douglas, which is far better written.