"Movies and pictures of Indians on calendars gave me a general idea of what
to do, and I waved the paddle slowly through the water, down and along the left
side of the canoe. The nose with Drew in it--I saw now that I was moving
him to one side or the other, to turn the canoe, was going to be a big part of
the problem--swung heavily out toward midstream, where the current began to pick us up and move it a little faster. The sensation of pure riding could not have
been greater though we were doing not much more than drifting, bogged with the
weight of the gear, and with uncertainty."
That's some good stuff, right there. The book, as you might imagine, fills in all of the gaps left behind by a two-hour screenplay. The first chapter takes you behind the motivations for taking the trip in the first place, which admittedly, the dialogue of the film takes care of less-than-adequately. The reader finds out Ed's last name (Gentry), for example, which is never mentioned in the movie at all.
But, this is one time when seeing the movie will not spoil the book. In fact, I recommend seeing the movie first and then reading the book. There are hardly any inconsistencies between the two ("Squeal like a pig" is not in the book), and the cinematography of the canoe trip adds to the mind's eye when reading about it. Since James Dickey wrote both the movie and the book, you can consider the movie sort of a condensed finished draft with a little added material.