If you're looking for information about Mail Art, here are a few places you might find useful:
Wikipedia, of course.
http://en.wikipedia.org/wiki/Mail_art
This will give you a nice, concise run-down of the world of Mail Art. Wikipedia is an online encyclopedia, so it isn't always the most entertaining. This is the typical Wikipedia page:

Yeah, I know: snoozefest.
I'll save my favorite as far as "fun to read" for the end.
MailArt.org
http://www.mailart.org/
This site is great for finding mail art calls. It also has all sort of fun little facts and information about what mail art is and where you can find it.

Wilson Web
http://vnweb.hwwilsonweb.com/hww/shared/shared_main.jhtml?_requestid=46293
These are some articles that I found about Mail Art. You can search Wilson Web for whatever you want. The best way to get there is through the Moore College Library web page http://library.moore.edu and go to the Electronic Resources link. Basically this is a way of searching for all kinds of articles. Many of them will have the full articles right there online and ones that are not available might be available in the college library or in the Free Library of Philadelphia.

Post Secret
http://postsecret.blogspot.com/
OK, so this is just some other blogger's spot, but it is really cool. You may have even seen their books at Borders. It's pretty big now.

I always find this entertaining and worth a look every few days. It is basically just a place where people can tell their secrets through mail art. A great idea.