I had this come up this morning.  I first deleted the book I had just added in case one of them had caused it. - no help.
Deleted DSLibris and files and replaced them. - no help.
I had been renaming a few game files as they were a bit long and I realised I had also renamed my book folder and called it books.  As soon as I changed it back to book, all is fine.  Have reloaded all my books and all is running normally.
Just thought it may help someone else.