"What was the last (best) technical book you've read?"
or, more generally:
"How do you keep your knowledge up-to-date?"
It's amazing how many people never read a technical book since they finished school. And if you never read a book since you finished school and finished school ten years ago, you probably never heard about things like unit tests, design patterns, SOLID principles...
Response to comment:
You can downvote me if you like, but this is one of my favorite interview questions. Blogs, wikipedia, SO are all great sources for the latest high-tech news. But I don't think you can learn really complex subjects (like the stuff you find in Knuth's books) in full depth by reading blogs.
If I have to choose between two developers, where one shows this willingness to learn new complex subjects and the other doesn't, I'll hire the first one. Even if he or she wants more money. It'll pay off in the long run.