What is the best source of historical evidence to learn about how an event unfolded

First hand data is always the first preference as it is very reliable having come from someone who saw / witnessed the historical event happen first hand. If this is not available, second hand data is just fine, but less reliable. But yeah first hand data should always be the best source of evidence due to the reliability and accuracy of the results taken from a first hand data. 

1. the spread of reform movements aimed at perfecting american society