Mario Puzo had real inspiration of Italoamerican families that were in the mafia business. The Five Families of New York were the inspiration for the Corleone family in the novel.
Like movies and series, songs can also be studied with the purpose of learning about a topic.
Personally, I dont know much about Italy´s role in the Second World War. Maybe this isn´t going to be very usefull when understanding the plot of the novel, but it could be interesting to learn about this topic, specially because the characters live with the repercussions of the war.
ArchDaily is the biggest architecture website in the wolrd. I like reading news , information about architects all aroud the world, new projects, and interesting articles they post everyday.
Learning about the author before starting the novels is something that we do every year. Studying the context in which he grew up, his studies, among other information can help the readers understand the novel better.
As any other millenial, I spend a lot of my time in social media (specially Facebook) in order to check the news and see what my friends are doing.
Besides "The Godfather", there are decens of movies and TV shows that talk about the Italian Mafia and all the repercutions that it had in the world, and IMDb created a list of the best ones. Watching other sources and comparing them to "The Godfather" could be interesting...
The author of this article, Marge Bitetti, is an expert on Italian migration to the United States after the Second World War. I consider that learning about the situation that the characters of the novel had to live, is going to be really usefull in the future.