Really, Really Big Questions about Space and Time is a great book that goes far beyond most kids' books about space:
http://www.amazon.com/Really-Questions-about-Space-Time/dp/0753430193

Ditto for the Basher astronomy book:
http://www.amazon.com/s/ref=nb_sb_noss?url=searchalias%3Dstripbooks&field-keywords=basher+astronomy&x=0&y=0

My son was giddy when he found the Basher book because it covered the Kuiper belt and oort clouds. Definite nerd moment.