This happened to me too a couple days ago. Haven't had any issues before with over 100 books on my iPad and iPhone. Now I have two PDF files that have that and I cannot delete. Also, a book I downloaded a couple days ago seems to be corrupt. When I am reading it, every time I switch programs and come back to iBooks, it closes and switches to the first category.
Deleting iBooks and reinstalling it didn't work, neither did a reset of my iPad.