I assure you the Godfather book is not a good book. Francis Ford Coppola himself, who adapted it into a screenplay, described it as trashy schlock. The movie is infinitely better.
My personal tastes agree with yours - but there ARE folks who thought otherwise - as I recall it was on best seller lists for some time.
I was also never able to make it through the the original novel Planet of the Apes was based on - but in that case, I assumed it was loss in translation between the original French and English versions.
