When I was younger we had a copy of the 1985 Encyclopedia Britannica: nearly 50 black leather-bound volumes with gold filigree and gilded edges. The index alone took up over 2 volumes, packed to the brim with every subject under the sun. For a number of years it was the single nicest possession in our household, although I can't imagine that either of my parents ever read a single word in it. I would sit around as a child and thumb through the pages, reading articles on all sorts of subjects. However, though I would read the words and see the occasional image, I can't say that I learned a single thing. An encyclopedia is an excellent source of information, but without the necessary background and instruction, the information will never become knowledge.
4 years ago (Wikibooks is coming up on it's 4th birthday) some instructional material was written on Wikipedia. It can't be denied that textbook-style content doesnt belong on Wikipedia, and therefore this content was summarily nominated for deletion. This kind of content does have an intrinsic value, and so the powers that be decided to create a new project for it: Wikibooks. A textbook can do more then simply present the information, it can teach it, along with examples, demonstrations, exercises. We can write a module not only on a particular subject, but also with full consideration of the background information that the reader will have.
For an example of what I am talking about here, I suggest this challenge: Pick a random, but decent, wikipedia article on a science, mathematics, or technology, and try to read through it. Without the necessary background information, I would be surprised if you learn anything from that article whatsoever. This illustrates my point, that without the proper background information and instruction, the presentation of information is simply useless.