I'm with plawrence. The book was wildly popular in its day. But the film--especially Brando's performance--was what absolutely fixated millions of people on the GF story and on Mafia stuff in general.
Analogy: Margaret Mitchell's "Gone With the Wind" was a runaway best-seller in its day. But without the world-beating film, few people would have remembered the novel 30 or 40 years later.