Let's be honest. The W3Schools are one of the most famous sites... even if sometimes they're a bit outdated, they do whole a bunch of interesting information on different technologies.
I think they wanted to get advantage of their names and that's a good idea. The drawback is precisly that the website needs to update content as good practices are not always reflected. A titacnic work but that's what the site exists, never the less.
I hope that the books do follow best practices and recommendation, and that they do explain why you should do it one way or the other. But I'm with you and I don't know if that will be the case. That's something that you don't site on w3Schools.com so... maybe the book follows the same line of work