We are reading /either/ Clarke's first novel, "Against the Fall of Night" or his later, rewritten version, "The City and the Stars." Clarke intended "The City and the Stars" to replace the earlier version but both have remained in print because many prefer the earlier version.
For those who don't have enough time (though the books are short), there is the short story, Wall of Darkness, also by Clarke. Full text here: http://webcache.googleusercontent.com/search?q=cache:0VOoDF8h2zgJ:www.lunsfordnet.com/get/txt/27765+&cd=2&hl=en&ct=clnk&gl=us
Radio play version here: https://www.youtube.com/watch?v=ER5nxcY4eqE