IE 6 has a lot of issues when it comes to the box model (and many other features).
Personally, I wouldn't spend much time on IE 6 anymore (as you found out, I haven't, for this book):
http://www.ie6countdown.com/
If you do need to support IE6, make sure you have a valid doc type (to put the browser in standards mode) and read up a bit on the differences in IE 6:
http://www.google.com/#hl=en&sugexp=...6+css+problems
Cheers,
Imar