So I'm in a number theory course this semester but we aren't using a textbook (the professor gave the explanation that they'd never found a textbook that does exactly what they wanted or something, but it seems from my investigations that our course follows a pretty standard introduction). Anyway, I have a test coming up this week, and I think I'd like to find a good textbook to help me prepare. This is the second test, and after not performing as well on the first test as I would have liked, I believe reading something else would help. My notes are too disorganized to be of real use besides helping me remember concepts, and I obviously cannot write down detailed explanations of the "whys", only copy down the "hows." So if you know of a good book for this purpose, I would really appreciate any recommendations.