Joel's The Absolute Minimum Every Software Developer Absolutely, Positively Must Know About Unicode and Character Sets (No Excuses!) and Dan's What the heck is: A string both cover the what, why and how of Unicode. Me, I work in The City, and i18n just isn't an issue. UK English only.

The effbot, as ever, gives you everything you need to know, with added brevity: Some Observations on Working With Non-ASCII Character Sets.