Nightfall was originally a short story by
Issac Asimov, and later became a full
novel co-authored by
Asimov and
Robert Silverberg. It is the story of the planet of
Kalgash. Kalgash is unique because it is oribited by six
suns, the chief sun being Onos. Because of the 6 suns, there is perpetual
daytime on the
planet, with always at least one of the suns being overhead at all times.
The story beings with a psychologist named Sheerin being called to investigate the effects of total darkness on people who have lived their whole lives with light. This effect is very very bad, it makes them go insane. Events work out so that a small group of people on Kalgash discover that a dark planet is going to cause an eclipse one day, and has been doing so every once every 2049 years, and civilization has always collapsed every time. There is also a mysterious religious cult called The Apostles of the Flame. They have apparently survived at least a few cycles of darkness, and intend to fill the power vacuum left by it during this cycle.
So the darkness comes, madness ensues, and part 3 of the book deals with the characters on post Darkness Kalgash (the other two parts being "Twilight" and "Nightfall").
A fairly average book, the original short story is much much better. It probably should have remained in short story form. I've had bad experience with co-authored books, but this one isn't that bad, by the middle you don't really notice it anymore. The characters are all fairly boring, and the ending sucks. Its obvious from reading it that huge chunks of the book were borrowed from Asimov's other works, especially the Foundation Series and Robot Series, but with a few twists added. The ending seems to be ripped from a chapter of the Foundation Series, while the psychological stuff is a part of Asimov's Robot Series. Those books are much better. Read those, they develop the ideas much more fully, and are just better books in general. I wouldn't put Nightfall in a class with them.
Overall Nightfall is ok, parallels aside the concepts in the book are pretty good, even clever, just not mind-blowing. Thats why it does so well in short story format, but is not quite as well delivered in novel form.