Why are economics textbooks so expensive?

A short video exploring some of the reasons for rapid inflation in the price of college textbooks in the United States - but an issue with wider appeal to students here! The textbook industry is an oligopoly - almost 80% of the textbook industry is dominated by 5 publishing companies including Pearson.

Has the rise of self-publishing by independent e-learning businesses (including Tutor2u) led to any noticeable change in pricing behaviour? How large is the market in second-hand textbooks? More generally, how many schools are eschewing textbooks completely in favour of more manageable consumables that students can complete themselves with guided learning?