In cover view, the number of covers shown is usually different in portrait versus landscape. Because of that, when you rotate the device CC often cannot display all the same books.
The rules that CC uses to display books after changing the orientation are:
- Before the orientation change, find the book displayed in the upper left corner (it might be partially displayed). Call this the "chosen book."
- After the orientation change, ensure that the chosen book is displayed somewhere on the first line.
There is no guarantee that the chosen book will be in the corner. If the number of books per row changes, it is almost certain that the book will move to somewhere else in the row.
Because the chosen book can and will move in the row, if you change orientation again then a new book will be chosen and the process repeats. The book previously in the left corner will move to the middle. As you change orientation over and over, the books being displayed will move slowly backwards as the chosen book changes.