A destination like Siem Reap is about as exotic as it gets for most tourists. Supplemented reading can definitely enhance a trip to this mysterious land filled with ancient temples of lesser known societies and religions.
"A History of Cambodia" by David P. Chandler should definitely help catch traveler's up to the beautiful and violent moments of this country's rocky history.  To help you understand the government control that Cambodia suffered, try: "The Pol Pot Regime: Race, Power, and Genocide in Cambodia under the Khmer Rouge, 1975-79" by Ben Kiernan. "Facing Death in Cambodia" by Peter Maquire also touches base with the violent past the country faced under the Khmer Rouge.
On a different note, exotic travels also mean exotic cultures. To help prepare for the customs of Cambodia, pick up "Culture Shock! Cambodia: A Survival Guide to Customs and Etiquette" by Peter North.
A travel guide will come in handy in Siem Reap, Lonely Planet Cambodia will definitely help. Once you've completed your journey here, you may want to know that you can tick it off as a destination to hit before you die as Siem Reap is mentionned in "Unforgettable Places to See Before you Die