Jim Holt reviews George Johnson's A Shortcut Through Time: The Path to the Quantum Computer. I haven't seen Johnson's book yet but the more I read about the more I recommend it to laypeople who are interested in quantum computers. It really seems like he gives an understanding of quantum computation without overly hyping the topic.
There is also an article from the circuits section on new work on at the Weizmann Institute on DNA computers. As typical with articles on DNA computing in the New York Times, you should read the end of the article first. That way you get the reality before the hype.