Recently finished re-reading Pierre Boulle's La Planete de Singes/Monkey Planet , the 1963 French novel upon which Planet of the Apes (1968) was based. I last read it as a confused 12 year-old, whom at that point had seen only the first two Apes movies and read the comic book magazine published by Marvel and Curtis. Reading it again as an adult has certainly given me a greater appreciation of Boulle's strange work. Perhaps the most interesting thing was recognizing elements of the novel which made it into not only the original Apes film, but its subsequent sequels, spin-offs and remakes. You can particularly see elements of the book which later ended up in Escape from the Planet of the Apes (1971) and Conquest of the Planet of the Apes (1972), while the novel's ending is actually more in tune with Tim Burton's maligned 2001 remake than those famous final moments of the original film, where Taylor (Charlton Heston) discovers the ruins of the Statue of Liberty half buried in the sand, the stranded astronaut realizing he has come full circle and landed on the nuclear ravaged Earth of the far-distant future (as in the Burton film, the novel does not indicate that the monkey planet is Earth, though it does reveal evidence of a technologically superior race of humans preceding the rise of the apes). Interestingly, the novel doesn't hint at any kind of natural or man-made disaster being responsible for the downfall of human superiority on the planet, but rather points to a gradual deadening of the mental capacity of the human mind coinciding with a rise of intelligence in apes, who (ala Conquest) have been trained as household servants and manual workers until they stage a revolt.
Ending aside, the other main difference between Monkey Planet and Planet of the Apes is the technology of the age in which the apes are dominant. While the film adaptation had the apes living in a very primitive society of horses and wagons and clay houses (guns and box cameras seemingly as advanced as they got), the apes of Boulle's novel were much more advanced, driving cars, flying planes, playing the stock market (!) and even starting to experiment with sending rockets into space. While the change in the film was an important one both thematically and cinematically, it's interesting that this more advanced civilization of apes was depicted in the short-lived Return to the Planet of the Apes cartoons series from 1975 (of course, a race of advanced apes was a lot more easier to visualize in animation than it would have been in live-action at the time).
Now to move on to Michael Avallone's paperback tie-in novelization of the second film in the series (and my favorite entry), 1970's Beneath the Planet of the Apes .