As a rough rule: implementation stuff ages. Design doesn't.
The basic principles of human-computer interaction haven't changed an awful lot since the days of MITRE's famous 1979 report on the usability of jet fighter computer systems. People still need to be able to discover content, recognize keywords, spot visual hierarchies and use alignment and common movement to identify element groupings. These requirements are based on simple human psychology, and that's something that hasn't changed much in a good 500,000 years.
Not convinced? Consider Nielsen's advice in his 'Top Ten Mistakes in Web Design', written back in 1996:
- Don't use frames
- Don't use technology gratuitously
- Avoid animations
- Keep URLs readable
- Avoid orphaned pages
- Make sure users don't need to scroll to discover content
- Help users navigate your site
- Format hyperlinks consistently
- Keep content updated
- Be wary of page load times
Of these, only #4 and #6 have lost relevance (no-one hand-types URLs today, and 2012's users are much more scroll-happy - though they still need to know there's a value to staying on the page before they'll spend a long time scrolling).
What does age, though, is advice that talks about implementation constraints. A book that tells you to avoid special fonts because image replacement breaks accessibility, for example, isn't really that relevant in the era of @font-face. Still, basic advice about bandwidth limitations and graceful degradation will remain just as relevant today as it ever would in the palaeolithic web.