I've noticed exactly the same thing myself. E-books are great for fun reading, but when I have to learn something I find it much easier on real paper. I don't own any kind of e-reader though so I can't speak to whether that helps or not.

I have no problem with linear reading books, novels , ... but this kind of books that i like to go forward and backward to reread i prefer paper ones, have same problem with recipes (cooking) ones.

I've noticed also noticed this. Currently I'm thinking of buying a cheap refillable laser printer to print out books an articles. There is something else about reading something on paper and being able to work through it (highlighting, taking notes etc.). I've tried PDF annotation programs, but none of them worked quite well.

I think the solution to this problem might be either ultra high resolution monitors or e-ink monitors.

I think it's just easier to skip around, not read it slowly.

Are you comparing paper to screen, or paper to eink?

I was comparing paper to screen. IMHO for skipping around monitors win hands-down, but for learning and extracting information paper provides a much-more focused experience.