I'm not sure I agree. It reminded me of certain "great" textbooks that sometimes get recommended by experts, that aren't actually that great for someone learning something for the first time. They get the reputation as being "great" because they are great for people that already have a good basic grasp of the material: They are precise and pithy, and they contain insights that are deep and useful, but hard to appreciate if this is your first encounter with the material. (Kleppner & Kolenkow would be a good example of a freshman physics textbook in this category.) It seems to me that maybe the ZeroMQ Guide is in this category. Which, of course, is a noble and important category, but maybe not the most important category if you're just starting out on a topic.