Why I cannot delete a page in iBook Author

I am editing a book. I have deleted some pages without trouble, but there is this page I cannot delete. Everytime I try to do it, iBook Author deletes the next page, not the one I want to delete! I have tried adding an intermediate blank page, but if try to delete the page I want to delete, it deletes the page next to the blank page!

The book was created initially by another author (unknown to me).