These were recommended to me. I am simply passing them on should anyone find them useful.
A tutorial on character code issues
http://www.cs.tut.fi/~jkorpela/chars.html
Excellent
Character Sets / Character Encoding Issues
http://www.phpwact.org/php/i18n/charsets
Pretty good overview of common issues but some gaps in coverage.
Has good list of resources at end too.