X-link to the Non-Fiction megathread since the new Humble Bundle has a whole bunch of kid-friendly computer/science/LEGOŽ-oriented books in it:
Bargain @ pay-what-you-want to $15:
the Humble Brainiac Book Bundle Presented by No Starch Press, containing a whole lot of LEGOŽ guides, and programming introductions for kids for the Ruby, Python, and JavaScript, and more!