I've got hundreds of art/design books, here a couple off the top of my head that I'd particularly recommend:
Type & Typography, Baines and Haslam
Simply a great, and fairly comprensive book about typography. Good reference if you unsure of a specific typographic convention etc, plus much more.
Grid Systems in Graphic Design, Josef Muller-Brockmann
A must have for obvious reasons really.
Design history books are a must as well really (too many to list here) but look into the Bauhaus, Futurism, Dadaism, Constructivism etc. Not only are these movements interesting they are also massively influential on graphic design/art/architecture and much more.
I'm more of a graphic designer and only really capable of building static CSS based web sites, but what I do know about web design has come from the XHTML book already mentioned by Bazzle, plus CSS Mastery, Web Standard Solutions (Dan Cederholm) and Designing With Web Standards, by Zeldman.
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