Knol is a wiki-based predominantly academic article writing platform promoted by Google. The platform has promoted writing textbooks on it. Textbooks can be written in various models on it. One model is to write it as one knol or one web page for each section of a chapter. That way a book can be written in around 300 pages and an automatic navigation is provided by the Knol collection feature.
There is convenience and reward for writing a textbook on Knol. The expected revenue earned can be same as print publication as there is global audience on Knol.
See the article on Knol for more details