I love the film but still haven't read the book. They say it answers all the questions we ask while watching the film (and can't be answered without reading the book).
AFAIK the book, which was written simultaneously with the screenplay, expands the description of the ending, which is where the vast majority of the questions come from.
The book explains what happened much better than the film, and gives more details. It's also less likely to induce a seizure.