Some of our books make use of streaming videos rather than embedding them within the iBooks files. As a result, you need to be connected to the Internet in order to access these videos. Furthermore, you also need to ensure that you have online content enabled in iBooks.
To enable online content, go to "Settings > iBooks " to see if online content is enabled. If not then please enable it and see if this works (note that some videos might take some time to load as they are large!).