There may be full-time writers who can finish a book of broad technical scope in several weeks. I don't know any who can afford to write technical books full-time, so I haven't anyone to ask. My own experience is closer to a chapter a month when I can "dedicate" myself, way less when life throws a curve or two. Remember that even if you are writing as an expert on your subject, you'll need research time, editing, diagraming and so on.

As for that demanding lot of editors, in my experience with Manning everyone has been very supportive through a series of delays with my work. My wife developed pregnancy complications, and those deadlines went wooshing past with barely a wave from me. Maybe I'm lucky to have an editor who has kids :)