Each learning format has its strengths. When I was first getting started with Photoshop, I did a lot of tutorials, and they made up about 95% of how I learned how to use Photoshop and still today, I am doing them.
Graphical tutorials might be different than coding tutorials. You really need to have Photoshop or Illustrator open while you read the tutorial to really learn something. For learning a new code you can read a lot before you actually put that knowledge into practice.
I still find I need to be on the computer with the code editor open while I do a tutorial. You can have your book next to you while you code, but it definitely isn't as easy as when a tutorial give you code snippets you can copy/paste into your editor.
Test What You Are Learning
Doing web design tutorials online also allows you to test what you are doing while you are doing it. It gives you an instant sense of accomplishment. You can validate what you are learning really works.
If you are reading a web design book, using another book to keep it open while you code is not very convenient, but you can still see what you are doing.
Web Design tutorials are generally more current and up-to-date. If you find tutorials to do on Twitter, chances are the author just wrote it. With a book you have to make sure it was at least printed in the same year you are in (the book I downloaded was printed 3 months ago so that is really recent for print).
Now so far it seems like doing web design tutorials online is the clear winner. It is convenient because you are at your computer, you can instantly test what you are learning and they are very current, so why would you buy a book on Web Design? It's simple, more content in one place.
Web Design Books Have More Content in One Place
When you find a tutorial to do you might have 800-1000 words in that tutorial. Once you learn what was written you have to do more searching for another tutorial to fill in the gaps left by the tutorial you just did.
Reading a Web Design book gives you sometimes 200-300 pages of content. Even if you jump around while you read, you can always go to the table of contents or index of the book and go directly to what you want.
Searching Google or Twitter for a concise list of everything you want to learn about web design can take a lot longer and you still might not find what you are looking for.
Which is Better, Book or Tutorial?
So which form of learning is better? I don't think we can actually declare a winner on this one. Each form of learning has its own strengths and weaknesses, so really it is a matter of the subject and how you learn.
Growing up I was always a magazine person instead of books, I like short concise bits of information. Reading a 400 page book was unthinkable, so for me tutorials work very well. This is not to say the book I just bought will go to waste. Why do you think I got the Kindle version? I can search it, view it on my computer, my tablet or my phone if I want. Really it takes one of the weaknesses of learning from a book out of the picture.
Really I think you should blend both learning formats to take advantage of each of their strengths.