Also anything by Martin Gardner, because his books are so much fun and help to spark your imagination.
At a young age one of the most important thing to develop is a habit of perseverance and not giving up when trying to solve a problem and avoiding developing areas of learned blankness. You should develop an unfaltering confidence to use your own head when trying to solve the problems. Sharpening mental capabilities and developing good mental habits and attitudes seems to be more important than learning more things (for example, the author of many AoPS books, Richard Rusczyk, thinks that it is better for kids to sharpen their minds solving olympiad problems than learn calculus), although desire to learn more, to build your own understanding, is also important. And it is not necessary that the problems are mathematical in nature. For example, if you read Richard Feynman's "Surely You're Joking, Mr. Feynman!", you would notice that as a young boy he loved to fix things and everybody brought their broken radios to him. He would then fix them, seeing it as a challenge, as a problem to solve. He had to find a way to fix it, no matter how non-obvious the problem was. I think this helped him to sharpen his mind and instilled a good habit to see interesting problems everywhere. If you have to think for yourself, you lessen the risk of developing learned blankness. Try to think for yourself, even if it takes much more time than simply finding solution on the internet. In the long run, developing good mental habits is probably the most important thing.
A few notes about the site mechanics
A few notes about the community
If English is not your first language, don't let that make you afraid to post or comment. You can get English help on Discussion- or Main-level posts by sending a PM to one of the following users (use the "send message" link on the upper right of their user page). Either put the text of the post in the PM, or just say that you'd like English help and you'll get a response with an email address.
* Normal_Anomaly
* Randaly
* shokwave
* Barry Cotter
A note for theists: you will find the Less Wrong community to be predominantly atheist, though not completely so, and most of us are genuinely respectful of religious people who keep the usual community norms. It's worth saying that we might think religion is off-topic in some places where you think it's on-topic, so be thoughtful about where and how you start explicitly talking about it; some of us are happy to talk about religion, some of us aren't interested. Bear in mind that many of us really, truly have given full consideration to theistic claims and found them to be false, so starting with the most common arguments is pretty likely just to annoy people. Anyhow, it's absolutely OK to mention that you're religious in your welcome post and to invite a discussion there.
A list of some posts that are pretty awesome
I recommend the major sequences to everybody, but I realize how daunting they look at first. So for purposes of immediate gratification, the following posts are particularly interesting/illuminating/provocative and don't require any previous reading:
More suggestions are welcome! Or just check out the top-rated posts from the history of Less Wrong. Most posts at +50 or more are well worth your time.
Welcome to Less Wrong, and we look forward to hearing from you throughout the site!
Once a post gets over 500 comments, the site stops showing them all by default. If this post has 500 comments and you have 20 karma, please do start the next welcome post; a new post is a good perennial way to encourage newcomers and lurkers to introduce themselves. (Step-by-step, foolproof instructions here; takes <180seconds.)
If there's anything I should add or update on this post (especially broken links), please send me a private message—I may not notice a comment on the post.
Finally, a big thank you to everyone that helped write this post via its predecessors!