
The book that tells of Master Chief's origins and is most relevant to all the games through Reach is Halo: Fall of Reach by Eric Nylund, the first Halo novel.
If you're really into the alien races that make up the Covenant, Halo: Contact Harvest was really cool because it told about humanity's first contact with them.
If you're really into Halo 4 though, I would say the Karen Traviss novels, Glasslands and The Thursday War are really fun because they tell of events going on right before Halo 4.
And if you want to know all about the Forerunners, read the Greg Bear books, Cryptum, Primordium, and Silientium.