Good question. I can tell you from my experience that I've learned a lot from the video tutorials on this site, especially as far as the AJAX COntrol Toolkit goes, as well as some of the newer features of .Net 3.5 (not so new now, but at the time...). I didn't think I could find a good book on those specifically, and even so, I don't think it would be worth it.
On the other hand, if I was learning ASP.NET from scratch, or C#, or SQL, or something like that, I think a book would serve me better. But best of all is practice (for me anyway).