As far as I can understand, there are two stories called "nightfall". One is approximately 80k in txt format, and was written by Isaac Asimov. The longer 700k-txt novel is based on this and written by Isaac Asimov and Robert Silverburg. I have here both versions, in various formats.

Also included, in a rather odd way, the front cover in GIF format. If you want it, download the utility CLIT.EXE and use it to extract the image from the lit file. In theory theres an HTML version you can get out the same way, but its in some strange character set.