In response to the directors comments, I would say that it focuses mainly on the first third of this book, because while the film focuses on the story of a father who pulled himself up by his own bootstraps, the book focuses more on the growth of the son. As the son matures he becomes his own person, so when he is young (as he is in the first third of the book), we see more of the father, because the son is more dependent on him at a young age.
The story also isn't about the growth of an oil firm at the hands of a maniacal of a father (as it seems to be in the film). For while we see the firm grow, Sinclair, the author, was more interested in portraying real life, and how employees are treated by big business at the time.
Had the film been made this year, I'd argue that the film would have a very different focus. Instead of focusing on one mad individual, it could have been a damning indictment of corporate greed and the way it treats its employees, which is something more like what the book is like I think.
In short, if the director had made the book, it would be a lot more challenging for big business to defend the actions of the central characters here, and so it is fair to say that this book is not "There Will Be Blood"