A senior friend of mine who is going to graduate school in mathematics suggested that I try to get at least some exposure to number theory before applying to/attending graduate school. (I'm a freshman undergrad.) Well, I was going to do so anyway, since it's interesting and even applicable, but now I need some direction. My university offers an elementary introduction to number theory at the undergraduate level. My friend has informed me that with some abstract algebra, the entire course will be trivial. I'd like to see what options there are for studying number theory in terms of textbooks, so I can maybe arrange some independent study with a professor or grad student. My aforementioned friend has suggested Rosen & Ireland's "A Classical Introduction to Modern Number Theory." Please suggest any books you have found useful, and please include the prerequisite background in mathematics/other sciences appropriate for each suggestion. Suggestions don't have to require any particular background knowledge; just fire away, as long as you include what's required to really benefit from the text. Thanks in advance to all responders!