With all due respect, a book is nothing like technical documentation. The biggest issue with documentation is that it needs a lot of structure. To do interesting things you need to be able to mark certain passages of text as special (e.g. this term is in the glossary; this block is a method signature), and you need hierarchical structure (e.g. you can't just have bullets points that appear more or less indented, you need some bullet points to be children of other bullet points). That's what Markdown is terrible at.
