Not so long ago I took a class called "Discrete analysis". I remember that I couldn't find any "novice" level material on Mobius functions in combinatorics. So then I went to the roots and read Rota's original paper "On the foundations of combinatorial theory I" and it really impressed me. So I wonder is there other mathematical subjects that it would be better for novice to get started with by reading rather original papers than actual books?

ADDED: Thanks for your answers. That's really interesting!