A book written by Drew Karpyshyn, who is also the lead writer for the video game. It is a prequel set 18 years before the events in Mass Effect, the game. The main character is David Anderson, who is also in the game (voiced by Keith David). The novel also gives reveals more on Saren Arterius, the main antagonist of the Mass Effect game.
You don't need to read the novel to understand the story of the game, because they both tell a different story within the Mass Effect universe. Though, if you are excited about the game, I recommend reading Revelation.
You can read sample chapters and order the book online at http://masseffect.bioware.com/galacticcodex/revelation.html