Knol is a wiki-based predominantly academic article writing platform promoted by Google. The platform has promoted writing textbooks on it. Number of medical books are started 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 chances of success of the text book are more on Knol as there is global audience for articles written on Knol.
See the article on Knol for more details