+1 for online resources as well.
On YouTube, I really enjoy David McPhail. He's a MARVELOUS tier, and a very good teacher too. It's nice to be able to see someone actually tie the fly. You can learn a lot about method and technique from watching someone that you can't learn in a book. Watching Davie's videos taught me how to properly whip finish a fly. There is NO way a book could have taught me that.
The nice thing about YouTube, or online in general, is that you can post questions, and generally get a response. Then those questions, plus others questions, are publicly available for all to read, further growing the knowledge base on the subject.
TCO Fly Shop's as well as Trout Predator Online (TPO) videos are very good as well, although TPO needs to do something about his audio.
One of the major benefits to online resources is the availability, both good and bad, of information. You can simply Google any recipe you'd like and generally you get a bunch of relevant information. With a book, you're limited to the printed recipes.
Another big factor, is cost. Online resources are free. And they're dynamic. You can find cutting edge patterns and materials online, while a book is stuck in a time bubble based on when it was published.
It's just my opinion, but I learned how to tie by only using online resources. I've never felt like it's hindered my abilities or haven't been able to find a recipe.