Here's a sample chapter about the history of the scene from freax.hu, which is a book attempting to summarize the history of the demoscene. Since the scene really became big on the commodore 64, this chapter focusses on the history of the scene on that machine.

Read more here:
Freax - The demoscene history project

Here's an old usenet thread from 1997 discussing the origins of demos. Of course in the end the opinions still differ...

Check it out:
Google Groups: View Thread "When was the first demo?"