The Universe in a Nutshell by Steven Hawking is a fun read.
I agree with this one and recommend it. Although I seem to recall that one or more of the theories in it are now out of date.
I also recommend How Computers Work
by Ron White and Timothy Edward Downs. I was looking for a very beginner computer book for someone, that started with the fundamentals, and this was the best I could find. I think that once you learn the concepts of how they work, it's much easier to start learning how to actually operate them and maintain them.