How about The [Theory of Remainders][1] by Andrea Rothbart. 

I remember back in the day I was struggling with the concept of modular arithmetic and randomly came across the book above. It's really well written in an unorthodox way as a dialogue between two people talking about modular arithmetic. The book introduces basic concepts of abstract algebra and has plenty of "simple, but serious" exercises. If I recall correctly, it did a really good job of motivating the concept of fields. Above anything, it was written with a high school audience in mind, so incoming freshmen should not be deterred by the level of difficulty. I also found the style of the book engaging. I dare say I was bitten by the number theory bug shortly after reading it. 


  [1]: http://www.amazon.com/Theory-Remainders-Dover-Books-Mathematics/dp/048644256X/ref=sr_1_1?ie=UTF8&qid=1296537579&sr=8-1