Ted Chiang's short story collection is the most concentrated dose of fantastic sci-fi/fantasy I've read. Each story is brief, memorable, and plays with at least one neat "what if?"

I've also given out a couple of copies because it's a great way to demonstrate SF&F genre awesomeness to the sceptical in just a few pages.

I've given about 30 copies of Kathy Sierra's Badass: Making Users Awesome. It's wonderful.

edit: whoops meant to reply to OP

I will probably give _Seveneves_ this christmas.