What about "The Little Book of Bigger Primes" by Ribenboim (see [1] for the Amazon link)? I personally think this is a great introduction to the field of number theory and I have enjoyed it very much a few years ago. It is clear and nicely written.

(Just to be clear: We are not talking about a course that also involves notable parts of algebraic number theory, are we?)





  [1]: http://www.amazon.com/Little-Book-Bigger-Primes/dp/0387201696/ref=sr_1_1?s=gateway&ie=UTF8&qid=1285070590&sr=8-1%20%22here%22