Following on, more or less, from the first Mass Effect novel (Revelation), this book moves forward a few years and finds Kahlee Sanders now working at Grissom Academy, a space station dedicated to the teaching and understanding of biotic powers, particularly in children.
One child possesses especially strong biotic powers, but all is not as it seems as the shadowy pro-human group Cerberus are clearly having a hand in her development. Moving at a brisk pace, the story follows the child in question, Gillian, plus Kahlee and others as they attempt to escape from Cerberus and Paul Grayson, Gillian's adoptive father and a Cerberus agent.
There are plenty of books based on video games nowadays, but the Mass Effect series is a little cut above the rest. Well written by Drew Karpyshyn, a BioWare writer in the first place, they have a really good feel for the universe and tie in pretty well with the games. Whereas Revelation primarily focused on an event alluded to in the first Mass Effect game, Ascension has to strike out more on its own. There's plenty of action, lots of backstory to various components of the ME universe that you encounter in the games, and well-rounded characters.
If you enjoy the games then this is well worth a read, even if you haven't read any other Mass Effect books. Although the book wraps up nicely, I highly recommend you pick up the next in the series, Retribution, which continues the story to a large degree.