This is the second part in my series of how to write a good wikibook, this one focused on the proper selection of a book's scope. The scope is, in a nutshell, the amount of "stuff" in the book. The scope defines for us what the book will include, what it will exclude, and to what depth the material will be covered.
One of the key concepts when considering the scope of a book, and the one that most wikimedians will be familiar with, is the idea that "Wiki is not paper". A wikibook simply doesnt have a maximum nor a minimum number of pages, and it is just as easy (and cost efficient) to write two short wikibooks as it is to write one large one. In the world of dead-tree publishing, books come with a pricetag--and that price tag is typically outrageous. We can't afford to buy two separate books, and so we prize books that cram in as much information as possible, even information that is loosely related or unnecessary.
Real-world books are designed for real-world needs: textbooks are typically broken up according to school semesters. Consider the school of economics, where first year students typically need to buy a book in "introduction to economics". The following year, you are buying an "introduction to microeconomics" and an "introduction to macroeconomics" book. To make matters worse, both of the two new books you have just bought include refresher chapters on the basic introductory material, as well as introductory chapters for the other discipline. At this point you have spent 300$, you have 3 copies of introductory material, lots of material overlap, and very little new information.
Being that a wikibook isnt paper, why shouldn't all this material be arranged more logically? instead of having an introductory chapter in each advanced book, you can post a link that says "read this first". Instead of having a series of books that are "introduction to...", "intermediate..." and "advanced...", you could have a single book with a single fluid progression of the material from the beginning to the end. This means there are fewer overlaps, fewer gaps, and more coherency between separate semesters of study in the same subject. Instead of having a whole pool, we can have three books on economics: "Economics", "Microeconomics", and "Macroeconomics". Or, if we were feeling ambitious, we could write just a single book with the knowledge that our book can be as long as we want it to be: "Economics". Of course organizing a large book that covers multiple sub-subjects and caters to multiple audiences can be an overwhelming task.
There is no one right way to do it, but that doesn't mean that some ways aren't better then others. Books, like many other areas of life, strongly benefit from a little bit of planning before you start putting pen to paper (or even finger to keyboard).