Complex sentence: Since they are primarily historical fiction they should be understood and enjoyed as such since the authors' opinion and those of the other primary sources or oral narrators of the stories he collects and transmit us as well as hearsay play a heavy role in his stories.
Split into simpler sentences: Since they are primarily historical fiction they should be understood and enjoyed as such. The author's opinion and those of the other primary sources or oral narrators of the stories he collects and transmits as well as hearsay play a large role in his stories.