It is possible that the memory on the Bebook reader is full. In that case, remove some books from it (using a file manager sush as Windows Explorer or Finder to view the reader as a mounted drive).
Also possible that the memory on the reader is corrupted somehow. In that case you may need to do a factory reset on it (check its instructions). Copy all the books already on it to your PC/Mac with the file manager first, unless you already have safe copies.