Fiction for an 11 year old.

[Cognitive Dissident]

The Walton Street Tycoons
The Adventures of Jonathan Gullible
The Moon is a Harsh Mistress

[DreadPirateRuby]

Hands down the best young adult libertarian fiction for young Adults is Tarrin Lupo's books.  He has 4 that are appropriate for that age.

The first is the Young Adults Pirates trilogy found here. http://www.amazon.com/Pirates-Savannah-Young-Version-ebook/dp/B006ITGRZY/ref=sr_1_1?ie=UTF8&qid=1324319566&sr=8-1

The forth book would be great if you homeschool your kids.  It is about of unschooling treasure hunters. It is called "The Hunt For Shamus's Booty" and you can find it here.
http://www.amazon.com/Pirates-Savannah-Shamuss-Booty-ebook/dp/B005WKGORG/ref=sr_1_1?s=digital-text&ie=UTF8&qid=1324319687&sr=1-1

Hope this helps! 

[The Muslim Agorist]

It's already been said, but the Hitchhikers Guide the Galaxy is excellent, and I'd recommend the whole 5 book trilogy and everythinkg else by Douglas Adams. They are hilarious, and I think they tricked me into learning critical thinking.

If you have trouble getting a kid to read a book their parent recommends you can do what my parents did. They asked me to read the first five pages and if I wasn't hooked I didn't have to read it. I read the whole first book in a weekend. The first book I ever read voluntarily.

I'm not sure if it's right for an 11 year old, but within a couple years the works of Robert Anton Wilson are excellent too.

[creighto]

I want to thank everyone for the suggestions.  My daughter read exactly one paragraph of The Moon is a Harsh Mistress before declaring it too difficult to understand.  I don't buy it, but I'm not going to make her read something that she doesn't like.  But per suggestions, I downloaded The Hunger Games to my kindle so that I could read it, and I thought it was awesome.  She's tearing that one up right now.  I'll be moving on to The Hitchhiker's Guide books next.

[SethCohn]

I want to thank everyone for the suggestions.  My daughter read exactly one paragraph of The Moon is a Harsh Mistress before declaring it too difficult to understand.  I don't buy it, but I'm not going to make her read something that she doesn't like. 

MIAHM is way beyond a 11 year old... the young adult books of Heinlein are her speed, not that one.  Give her Podkayne of Mars, or the other juvvies: http://en.wikipedia.org/wiki/Heinlein_juvenile

But per suggestions, I downloaded The Hunger Games to my kindle so that I could read it, and I thought it was awesome.  She's tearing that one up right now.  I'll be moving on to The Hitchhiker's Guide books next.

I just read Hunger Games, and very anti-authoritarian, I'm shocked they are making a movie of the first one...

[Kelvin]

It was Heinlein who did it for me.  As many have said, check out his "juveniles"

2 of the first I read were "Have Spacesuit, Will Travel", and "Space Cadet."  I credit Heinlein as giving me my first push in the direction of libertarianism.

Have to warn you, at the time Heinlein wrote these, he was what I could call a "Right - Libertarian" in that he was all for self-reliance and individual responsibility, but he was also a bit nationalistic.  Still, the trade off is well worth it IMHO. 

I found the stories very compelling and full of adventure, and promote self-reliance. self-determination and competence.

-Kelvin