The question came up yesterday: "How big does a book need to be?". Or, if I may rephrase, "What is the minimum size necessary for a Wikibooks module to be a book?". Surprisingly, there were some disagreements among wikibookians about the answer. It's not necessarily a bad thing, however, that we haven't codified every last detail. One thing that Wikibookians really pride themselves on is that they do keep the rules to a minimum, and they keep editorial freedom to a maximum.
Some people say that a book should be considerably longer then a corresponding Wikipedia article. I'm of the school that it's not a quantitative difference we should be shooting for, but instead a qualitative one. In my mind, what most separates out a book from an encyclopedia article is that a book should be instructional. Books likely should be more comprehensive then an encyclopedia article, but that's just a guideline.
People view book writing as a daunting task, but I don't feel like that is the case. Books can be long or short, in-depth or just scratching the surface. You can pick a subject niche, and expect other books to fill in the background information and other books to carry the baton into the more advanced subjects. On top of that, we don't have the same style and formatting guidelines that Wikipedia has, so authors are more free to get creative, or even go completely minimalist.
In short, writing a book should not be any more daunting then writing an encyclopedia article.