# Number theory for a high school Calculus student?

I've always loved playing with numbers, but haven't had any formal guidance in the study of advanced mathematics and number theory. Is there a book (or a few books) on mathematics that I wouldn't have learned in my high school curriculum (algebra, trig, and calculus) that could help me understand higher level maths and get me started on number theory? I know it's quite a broad question, but when I see things like this xkcd, and in trying to understand concepts I go to Wikipedia articles to find them chock full of terminology that flies over my head, it's quite frustrating.

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rutherglen.ics.mq.edu.au/wchen/lnentfolder/lnent.html is a good set of (free) notes on Number Theory. – JavaMan Nov 27 '11 at 7:03
The Art of Problem Solving Volume 1 and 2 (especially 2) are nice books which go beyond the standard high school curriculum in general and venture into competition math. They got me interested in higher math. – Shayla Nov 27 '11 at 7:08
If you're asking about book for self-study of number theory, you can find something in related question: math.stackexchange.com/questions/1774 (In fact, Stein and Underwood Dudley, which were suggested by Arturo, were mentioned there too.) Maybe you can have a look at questions related/linked to that one too. – Martin Sleziak Nov 27 '11 at 7:13
When I was in high school and looking for extras, I stumbled upon, and immensely enjoyed, Joe Roberts' Elementary Number Theory - a Problem Oriented Approach. His book has a very unusual style. All the theory is developed as series of exercises. The first half of the book is only definitions and exercises. The second half has hints/solutions/historical remarks. It was a perfect fit for me. Warmly recommend it for self-study. – Jyrki Lahtonen Nov 27 '11 at 9:45
@JyrkiLahtonen, you didn't mention the most unusual thing about the Roberts book - it's all done in calligraphy. – Gerry Myerson Nov 27 '11 at 11:40