Two competing factions of an alien race, using Humans as hosts to get off this planet, but with different philosophies [though not always that easy to tell the difference]. Now imagine you are an over-weight software engineer, getting in your car and one of these aliens merges with you and over a relative short time turns you into a secret agent in their war. This is the beginning of this book by Wesley Chu.
A good sci-fi book where the concept of what would happen in aliens were behind the leading figures in history is re-explored. Basically anything we think of us humans developing that is significant, is actually developed through these aliens. Now, throw in martial arts action, deception, politics, and blood-feuds what you end up with is an interesting and more importantly enjoyable read.