Properly hande user trying to add .kobo books to calibre from device

src/calibre/devices/interface.py

@@ -52,6 +52,11 @@ class DevicePlugin(Plugin):
     #: long time
     OPEN_FEEDBACK_MESSAGE = None
 
+    #: Set of extensions that are "virtual books" on the device
+    #: and therefore cannot be viewed/saved/added to library
+    #: For example: ``frozenset(['kobo'])``
+    VIRTUAL_BOOK_EXTENSIONS = frozenset([])
+
     @classmethod
     def get_gui_name(cls):
         if hasattr(cls, 'gui_name'):

src/calibre/devices/kobo/driver.py

@@ -38,6 +38,8 @@ class KOBO(USBMS):
     EBOOK_DIR_MAIN = ''
     SUPPORTS_SUB_DIRS = True
 
+    VIRTUAL_BOOK_EXTENSIONS = frozenset(['kobo'])
+
     def initialize(self):
         USBMS.initialize(self)
         self.book_class = Book

src/calibre/gui2/actions.py

@@ -430,6 +430,20 @@ class AddAction(object): # {{{
             d.exec_()
             return
         paths = [p for p in view._model.paths(rows) if p is not None]
+        ve = self.device_manager.device.VIRTUAL_BOOK_EXTENSIONS
+        def ext(x):
+            ans = os.path.splitext(x)[1]
+            ans = ans[1:] if len(ans) > 1 else ans
+            return ans.lower()
+        remove = set([p for p in paths if ext(p) in ve])
+        if remove:
+            paths = [p for p in paths if p not in remove]
+            info_dialog(self,  _('Not Implemented'),
+                        _('The following books are virtual and cannot be added'
+                          ' to the calibre library:'), '\n'.join(remove),
+                        show=True)
+            if not paths:
+                return
         if not paths or len(paths) == 0:
             d = error_dialog(self, _('Add to library'), _('No book files found'))
             d.exec_()