IMHO, i would say depends on your role, are you a designer or you're the coder guy who have to put functionality into the site?
if you're into the design, then i would say css and good website designs are necessary.
in that case i would recommend don't make me think and erik meyers css book.
but if you're into the functional part of web site, i would say learn javascript then jquery. i can't tell you how many times jquery save my bacon.
and finally nothing beats putting theory into practice, so you can either search around and view source and learn from good sites, or start creating your own :)