Lightroom :: Using Ebook Text For LR4?

I have been using the ebook text Classroom in a Book for Lightroom 4.  I am working on two screens:  Classroom in a Book is on the secondary screen; LR4 is on the main screen.  I have gone through 4 of the lessons.  At some point during Lesson 4 I must have done something and I can't figure out how to undo it.  What happens is that Classroom in a Book disappears from the secondary screen when I move the curser from the secondary screen to the main screen where I work on LR4 exercises.  What appears on the secondary screen is either a LR4 photo or a blank LR4 screen that says "No Photo Selected".  I can get back to Classroom in a book if I move the curser to the left margin and click.  But then I have to do this every time I move the curser from the secondary screen to the main screen.  How do I undo this so that Classroom in a Book remains on the secondary screen as I go from one screen to the other screen.