Comment by zyklu5

As a first pass, check out the pop history book called Fearless Symmetry by Ash and Gross.

Next, and perhaps I shouldn't be suggesting it, Serre's A Course in Arithmetic. Despite it's reputation of terseness it is a great book by one of history's great mathematicians and worth every sweat and tear spent over its short number of pages.

Yet another way is to see the lectures by Richard Borcherds (who won the fields for the moonshine conjecture) on youtube.

Finally, since this hacker news check out William Stein's Modular Forms: A Computational Approach (pdf online)