This is one of the best books that I have ever read along with Halo: The Fall of Reach. It really sets the scene, with much more detail in it about Halo and the Flood from all perspectives including human, covenant and the creatures unfortunate enough to be possessed by the Flood. The book tells the story of the game but with an almost 'behind the scenes' style of presentation. If you enjoyed the game, you will definitely love Halo: The Flood.
All in all, buy this book and it'll make you eager to play the game again just to find out more.