Jakob Nielsen's useit.com
Nielsen's site contains lots of thought-provoking articles on the subject of usability, some of which relate to navigation. The site is quite opinionated, and not everybody agrees with what he says, but his articles often make interesting points.
IA guide: navigation
http://argus-acia.com/ia_guide/subject/navigation.html
Many articles about navigation from an 'information architecture' point of view. (Information architecture is a common term for the kind of subject discussed in this and the previous lesson.) Most of these are rather scholarly, but some may be useful for beginners too.
