There Ain’t No Such Thing As Plain Text.
The Absolute Minimum Every Software Developer Absolutely, Positively Must Know About Unicode and Character Sets (No Excuses!), by Joel Spolsky.
Want to better understand why the content-type tag is important? Want to know the difference between UTF-7, UTF-8, ISO-8859-1, and so forth? What to know why that there is no such thing as plain text? Joel has answers for you.