The key to this problem is that readers cannot be charged for accessing Web pages. This forces the information provider, rather than the reader, to pay for the material. While some academic journals charge authors to print their work, this is unlikely to work in the textbook world. Unless Web sites can encourage sales of related books, they provide no revenue. This situation will soon change.
Web sites can therefore charge users, and so fund their material.
Publishers could conceivably charge by the Web page - perhaps finally breaking the 'bigger is better' view that grips the industry. This, however, ignores the absolutely fundamental challenge that the Internet brings to conventional publishing. The fundamental difference is that atoms exist physically, whereas bits do not. The relevance here is that printing and distributing books costs money. Producing an extra book costs money.