There’s a tendency to believe that all information is equally valuable. Therefore, an hour spent reading about, say CoffeeScript in blogs is equivalent to an hour spent reading a CoffeeScript book, only the blogs are free, immediate, and more entertaining.
The last bit is the key. They reason that blog posts are more entertaining is the reason to read the book. They are all sizzle, with optional steak. A book has structure, as Avdi notes, and part of that structure is balance between crazy-fascinating things and fundamentals that are not that fascinating at first but necessary for deep learning.
In cliche terms, books contain information that is important but not urgent. Thanks to the social mechanics of blogs and social news sites and retweeting, the blog posts the filter through the firehose are all urgency. Some also contain important topics, but none contain the important but not urgent stuff that is necessary for a complete education. Any blog post or essay with important but not urgent information falls into people’s “read this later” bucket and never gets the upvotes to make the front page of Hacker News or anything else.
So, you will learn things in an hour on the web, and in an hour reading a book, but if your primary goal is a thorough education in something that is new to you, an hour in a good book will serve you better than an hour reading blog posts.
p.s. Alas, my own books at http://leanpub.com/u/raganwald are simply collections of blog posts massaged into book format, and as such they are entertaining but do not provide the kind of structure and “important but not urgent” information Avdi and I are extolling here. For example, my essays about Combinators and Ruby might make you interested in Combinators enough to read a real book about them, but the book won’t actually teach you introductory combinatory logic.