The original 1968 film of Planet of the Apes was an adaptation of the 1963 French novel La Planète des Singes by Pierre Boulle and pretty much all of the 1968 film was a fairly radical rewrite of the Pierre Boulle book by Rod Serling to make it more dramatic and action-packed; remember books are more about thoughts and ideas and films are about actions. This Rod Serling website has a fairly great analysis of the work Rod Serling and other’s went through to rework Pierre Boulle’s novel into a film:
In late 1963, Rod Serling was hired by King Brothers Productions to
write a screenplay based on Pierre Boulle's novel Planet of the
Apes. For more than two years, Serling, who had earned a solid
reputation as a television writer, struggled with the task of adapting
this complex story for the big screen. By the time he submitted a
final draft in early 1965, APJAC Productions had acquired the screen
rights to Boulle's story. For the next two years, producer Arthur P.
Jacobs worked to raise enough funding for what had developed into a
very expensive project. Before filming began, another experienced
writer, Michael Wilson, was brought in to work on the script. Wilson,
whose career suffered through the blacklisting of the McCarthy era,
had written many excellent film scripts (including It's A Wonderful
Life and A Place in the Sun)—some uncredited until recently (such
as Bridge on the River Kwai and Lawrence of Arabia). Finally, in
early 1968, Planet of the Apes was released, with both Wilson and
Serling sharing screen credit.
So when all is said and done, the ending of the 1963 Pierre Boulle novel—as well as much of the story itself—has very little to do with the ending of the 1968 film adaptation. And the film and television sequels that followed the 1968 film adaptation have even less in common with its source material as the series went on.
So you are 100% safe to go ahead and read that novel without fear of one ending “spoiling” another. If anything you will get a deeper insight into the core DNA of the whole Planet of the Apes franchise before it even became a franchise.
FWIW, the ending you have alluded to in your spoiler text was from the second film in the series: Beneath the Planet of the Apes (1970). But—thanks to the use of “time travel” and the producer’s desire (aka: $$$) for sequels—the franchise continued with three more films and a TV series:
- Escape from the Planet of the Apes (1971)
- Conquest of the Planet of the Apes (1972)
- Battle for the Planet of the Apes (1973)
- Planet of the Apes (1974 TV series)
So yes, the sequential, in-universe Planet of the Apes timeline “ends” with the event you describe happening at the end of Beneath the Planet of the Apes. But the film series continued past that film and even into a TV series as well.
>! Your spoiler here; more details can be found here. As for the down vote—not given by me—honestly you are asking for a personal opinion on a series that is very well documented since before the Internet existed. I suggest checking out this article for more details. But then again, you don’t want a spoiler but you do want someone to validate what you perceive as a “bad” ending is somehow not in the books.