Two kinds of 'don't know'
I don't know French. I can't play the piano. I have no clue how to catch a bony spinefish. This is the first kind of don't know. Stuff you don't know because you haven't been taught it yet. Books are awfully good at solving this problem, so are good teachers.
The second kind of 'don't know' is often confused with the first type, but it's really quite different. This is the person who says they don't know how to cook, or that they can't balance a checkbook. This isn't about technique or a lack of knowledge. It's usually either fear or lack of interest. People with this type of deficit won't find the answer in a book or (usually) in a seminar either. You don't learn how to cook from a cookbook.
The answer lies in trial and error and motivation and in overcoming the fear that makes us avoid the topic in the first place.
And why should a marketer care?
You need to care because if you try to solve the second kind of ignorance with a manual or a PDF or a blog post or even a long infomercial, you're going to fail. If you discover that users are afraid or resistant to what you're trying to get them to do, more information is almost always the incorrect response. The effective technique involves peer pressure and support and in changing the design and inputs of what you're doing so that this group is more receptive to what's on offer. For example, internet penetration isn't up by a factor of 20 because people read a lot of copies of Internet for Dummies. It happened because of what peers said to each other over time, and because the act of getting online is a lot easier than it used to be. And you can help that happen.