While David’s articles at his blog are frequent and in-depth, any one of them is too tactical for a good overview. The article at the Innovate is a good summary and is rather more strategic. It also utilises the online nature of the journal to provide a comprehensive set of relevant hyperlinks. There is even a recorded Webcast connected to the article with further discussion on the issue.
(_Disclosure: I have previously written an article about e-books for language learning for the TeleRead blog)