The Prestige is a very good novel, and yet, the movie differs from it considerably. And I am still trying to figure out what exactly that means. The central premise is the same, AND HERE BE SPOILERS for both the film and the book, and yet key facts are very different.
The film ups the ante. In the novel Borden accidentally causes the death of Angier's unborn child and doesn't know it. In the movie his arrogance causes the death of Angier's wife and he is well aware of it. In the movie the "Pact" which allows Borden to do his spectacular trick leads to his wife's suicide. In the movie the women in Borden's life (or lives) seem perfectly content and unaware (a point that some characters cannot buy and neither can I). In the movie Angier injures Borden's hands--as important to a magician as to an artist or musician. In the book they merely disrupt each other's shows. In the movie, Angier frames Borden for his (Angier's death), when he is not "really" dead and knowing that only one of the twins will die. In the book, Borden does accidentally cause Angier's "death" and then Angier accidentally causes his.
All of these could be attributable to making it more exciting for the movie as could the discarding of the "modern" story line with the descendants of both men, but the ultimate change is that in Angier's version of the trick where Tesla's machine causes a duplicate to be made, in the book the original dies, like a shell that is discarded, and in fact, the bodies do not rot or decay--but are merely inconvenient to dispose of. In the movie--and this is the crux of whether we like either of the characters ultimately--Angier MUST kill the double each and every time the trick is performed--much more horrific and altering for Angier.
Of course, the underlying point/message of both is where obsession takes you--the half life of the Borden brothers, or the supernatural/superscience and ultimately soul killing answer of Angier. The final moments of the novel, when it seems that Angier has cheated even death with Tesla's machine--is nowhere to be found in the movie. Likewise, the careful presentation of the death and destruction (of birds and other animals) inherent in magic tricks of the time is carefully spelled out as a foreshadowing technique in the movie by Michael Caine's character and is not in the book.
Now, apparently, according to IMDB, Sam Mendes approached Priest about adapting the book but Priest only wanted the Nolan brothers to adapt and direct based on Memento and Following. So they took his book and made something completely different with the same materials--not better, per se, but very different.
How does one do that? How does one adapt a book at all, but especially to change it so fundamentally? Some books are considered unadaptable, and some directors (and their teams) have made a practice of adapting just those books. Interestingly, Sam Mendes is one of those directors. Another is Phillip Kaufman.
Kaufman directed (and for the most part did his own adaptions of) The Right Stuff, The Unbearable Lightness of Being, Henry and June and Quills.
The first two considered too unwieldy and the last two being from diaries. I haven't read The Right Stuff, but I have read The Unbearable Lightness of Being, and while many would disagree, I think he did an amazing job adapting a novel whose narrative is so liquid and undefined. Likewise he did a very good job adapting the diaries of Nin and de Sade into a contained story line for an interesting and engaging film.
Now he had little to work with in terms of plot and so his movies are very different from these books, but the Priest novel has an abundance of plot which the Nolan brothers rewrote. I'm not quite sure what to make of that. How, as an author, Priest could be happy with the radical changes to his book--to create what his novel should have been? One of Nolan's favorite films is Blade Runner, and I have often referred to Blade Runner as an amazing film which bears little resemblance to it's source material in a way that diminishes neither. I'm not quite sure I can say the same here.
But at the same time, I respect the director/auteur's right to make a better film. Interestingly, often the more true a film is to its source, the less good it is as a piece of film. The BBC adaptations are excellent adaptations--they are seldom cinematically relevant. It is one of my sorrows with the recent adaptations of The Chronicles of Narnia. They are perfectly fine adaptations with minor changes to make the dialogue snappy and increase the action--Lewis was never a great describer of battles, etc., and yet I feel that they are somehow weaker than they could have been and I blame the director.
In contrast I consider Peter Jackson's movies of Tolkien both brilliant adaptations and amazing films in their own right--because he is a brilliant director AND because he was able to find the central piece and make it his own.
For all of their magic and creatures and Norse mythology, at their heart, TLOTR books are about a lost England. An England that Tolkien thought was gone in the chaos of WWI, in the industrial revolution (although, both he and Lewis wanted post-industrial amenities in their worlds, with pre-industrial impact). It is a love poem to England's mountains green. Peter Jackson's films are a love song to New Zealand's mountains green and it shows.
Which brings me back to my original point. The movie is more horrifying than the book, and I'm not sure if that diminishes the book, or increases it.