We have recently made a change to the site that affects how book modules are displayed in some courses.
The book module icon looks like this:
We were finding that on smaller screens, such as laptops or iPads, the content in the book modules was confined to a very small space which required constant scrolling up and down. Now when when the book module is clicked, content will open up in a popup window and can fill the entire screen. To exit the pop up window, just close the tab. *Depending on your browser, you may need to allow pop-ups.