Apparently Robert Zubrin is coming out with a second edition of his book The Case for Mars. I read the first edition years ago, and found it inspiring. It really gives you a feeling of "Let's go!" and "Why aren't we doing this?" If you haven't read it, I recommend it (assuming the second edition is as good as the first). It's full of neat ideas. Mainly it's about Zubrin's plan for the first several manned expeditions to Mars.
Are there other good non-fiction books about the future of space travel? I can't think of any right offhand.