I'm interested in reading Gelbart's book "Automorphic Forms on Adele Groups". I have a solid background knowledge at the first-year graduate level (I've passed quals), but I don't know what the prerequisites for Gelbart's book are. I've browsed through the book.
To be a bit more descriptive about my background:
I haven't taken a course in representation theory (beyond representations of finite groups) and would be interested in knowing where I can get the required representation theory background before reading the book.
In addition, I am unfamiliar with terms like the "spectrum" of the space of cusp forms, for instance.