Fingerprints Of The Gods this is perhaps the first read in order to grasp the initial concept of missing history. It is not a simple read but it is well written and documented.
The Orion Mystery: Unlocking the Secrets of the Pyramids This one is optional, if you want to get a closer look at the Pyramids. It is a very well researched book that keeps an open mind on the various possiblities.
Forbidden Archeology: The Hidden History of the Human Race Also optional after Fingerprints of the Gods however the controversial archaeological findings are quite amazing and very well documented.
Galactic Alignment A slightly different topic, the 2012 saga but written by the man who first got the word out, John Major Jenkins. Very enlightening.
Supernatural: Meetings with the Ancient Teachers of Mankind A great insight on what is coming up and what to expect now that 2012 has been associated to doomsday by Hollywood, something The Mayans would not agree with.
Inner Paths to Outer Space This could be seen as the next step, beyond the mysteries of our distant past. This refers to the other realms of reality from a Shamanism point of view.