If you're totally new to the world of accordions, Accordion 101 explains the basic types of accordion and gives some of the history behind them.
Hans Palm's Accordion Page. This is one of my favorite accordion sites out there (even if it's only "out there" via wayback machine now.) Useful info, interesting information, and a page of accordion jokes!
Accordion Revival. Everything I know about the guts of accordions and how to take care of them is stuff I've read from this incredibly detailed, expert site. This guy knows what he's talking about.
Got/want a piano accordion and don't know where to start learning? Here's the first 4 Palmer-Hughes books, designed to bring the beginner musician through the process of learning accordion from scratch. These are what I'm using! (Book 1) - (Book 2) - (Book 3) - (Book 4). And if those are too easy, then try for some fancy fingerwork. Here's a PDF of Hanon accordion exercises.