You can select more than one title at once in the Library by holding down the Ctrl or Shift key while clicking on a resource. But once you've got a multi-book selection, you can't seem to do anything with this grouping other than right click and prioritize them all or open them all at once. If you try to click and rag (e.g., into a collection or into the "Hidden Resources" box on the Program Settings tab), it only drags the one you clicked on, even if a whole bunch were selected.