Hrm. There's a category 2.5 of "advice which is common knowledge to long-standing members of a small subculture but not to the public and that isn't written down." These are cases where the information isn't secret, but is primarily conveyed word-of-mouth, not in print, and where it's too specialized to be "conventional wisdom".

It might be that there's some piece of advice that most expert, say, patent lawyers would give you, but that wouldn't be in a standard book because the topic is too narrow or esoteric.

In my case, I've gotten valuable career advice from senior members of my profession. I don't think it was a unique boon to me and their advice parallels conventional wisdom, but with details that are specific to my narrow field.