Mass Effect: Revelation is a fairly straighforward prequel, filling in the back story between Captain (then Lieutenant) Anderson and Saren. Without giving away any more plot than the back cover of the book, the basic setup is that an Alliance research facility is attacked by an unknown force and Anderson's team is sent in to investigate. It turns out that there is only one survivor, a woman named Kahlee Sanders (I'm assuming her name is pronounced Kay-lee) who left the base only a short time beforehand. She's the obvious prime suspect for the attack, but Anderson sees things differently. There isn't a huge amount of new information in this book that isn't already in the dialogue of the game, but it's nice to see an expanded back story. There is a strange unnatural compulsion of the author to name the fictional manufacturer of every weapon, piece of armour, etc. whenever introduced as though he is simply showing off his knowledge of the Mass Effect universe, but this is a minor niggle.
Mass Effect: Ascension on the other hand, is a much more interesting novel. It is set directly after the end of the game and neatly avoids committing to any of the alternate endings to the game by following Kahlee (now working as a scientist in a elite biotic training school) as she is looking after autistic student Gillian, whose father is a member of a shadowy underground group manipulating the Alliance from within. He is highly conflicted in his dual role as an agent and a father and has become a red sand addict - a drug which gives temporary biotic powers which he uses (or at least started using) to try to better understand his daughter. The book shows us a lot of interesting things only briefly mentioned in the game, such as the Quarian lifestyle in the migrant fleet.
Together the books neatly bookend the first game in the series, and as short paperbacks are fairly cheap and quick to read too!
If you'd like to buy either book in the UK, I would be grateful if you could use the links within this article which will help pay for the hosting and bandwidth for this blog.