There are so many more details and people in the novel:
--The Don's rise to power in his war with "Maranzalla."
--Luca ordering the gruesome death of his newborn son.
--The Bocchicchio Family and how they provided Michael's passport back from Sicily.
--Sonny extending his protection to Long Beach.
--How Neri got recruited by the Corleones.
--Johnny's singer pal, Nino, and his brief Hollywood career.
--Dr. Jules Siegel and Lucy Mancini.
On and on.