The difference between a book and a web site is that any fool can throw up a tutorial. And no-one has to approve it before it goes on line, no-one has to check that it is correct, that it reliable and accurate, that it has enough variations to cover most of the things you will do, that it is up to date, that it matches modern standards (much of w3schools doesn't, although they are now, finally, trying to upgrade it).
A book however takes a bit longer to write (unlike a five minute tutorial), then goes to a publisher, where it may get rejected or examined in more detail. If provisionally accepted, it gets examined by technical editors for accuracy and quality, by proof-readers looking for typos and inconsistancies. Then the publisher has to decide if it is good enough and will go down well in the current market, when placed up against other similar books. Then the publisher has to invest in printing thousands of copies. So publishers prefer professional experts.
Online tutorials - well you could write one if you liked as your first web site, couldn't you.
The book I mostly use, is <Web Publishing with HTML and CSS> its by Sams Publishing. Usually has the answers to most of my HTML/CSS questions and has explanations and definitions for why you do things.