Creation and Duplication
You mention the salient point, mouse. It is only fair that the creators of intellectual property should be paid by the users of the same. For the distributors, it's a bit different. Producing a copy of a technical book with many graphs on dead trees once cost real money - far more than typesetting, and the books, with short print runs, were correspondingly expensive. Now that's no longer the case, so the price structure of textbooks (and sheet music) should rightly evolve to reflect technological progress. Strangely enough, that reflection appears quite faint in many cases.
For an editor like the house of Eugen Ulmer, the world has changed drastically in the last 20 or so years and I can understand they lag behind in adapting. Nonetheless, the first editors to find a universally accepted business model that enables easy duplication but provides a fair income per reader for the author and for the editor, will stand to make very serious money.
I have some volumes in the rack next to me that cost nearly as much as the laptop I'm typing this on, but most of that really goes into production and distribution; if I could buy newer electronic copies for 20% or so of their price (whatever reflects the fair cost of electronic publishing), I almost certainly would; and I guess the total number of electronic sales would dwarf the printing run.