Here are some of Twain's essays/short stories that I particularly recommend:

"The Awful German Language" will be enjoyable to anyone who has tried to learn a foreign language.

"Buck Fanshaw's Funeral" is all about dialect.

"How I Edited an Agricultural Paper"

I especially recommend reading the stories aloud. It's especially fun when you're laughing so hard you can't talk!

This seems to be a very good website: Mark Twain, Edited by Jim Zwick. Practically all of Twain's writings are available online, and this site has links to them.