While a recent guidebook on Panama is always useful, for those who want to understand more about the historical background of the places they are about to visit, the following two books are excellent reading.

Both "Path between the Seas" (by David McCullough) and "Panama Fever" (Matthew Parker) explain the fascinating history of the Panama Canal, the reason this country was 'created' in the first place.  When you visit the Panama Canal and surrounding areas after reading one of these books, you'll look at it differently.

Another good start is the Panama section on Wikipedia.