Based on this description, the princes work is a secondary source. A secondary source is a document or recording that relates or discusses information originally presented elsewhere. Secondary sources involve generalizations, analyzes, interpretations, or evaluations of the original information. They describe, discuss, comment upon, summarize, and process primary sources.
A secondary source is generally a step removed from a time period and is produced after the fact with the benefit of hindsight. Secondary materials can be written or non-written.