A friend of mine is a very competent graphic designer who wants to expand their skills and get into web design (just the visual design, no coding). While some skills certainly transfer over, web design also has its own specific disciplines that a print designer may not be familiar with (user experience/interaction design, web accessibility, differences in formats etc.).
So what would you recommend as the best books/web sites/other resources for a graphic designer looking to get into web design? General introductory resources would be excellent, but if there's anything out there specifically covering graphic designers looking to transfer their skills over that would be fantastic.