[SOLVED]Cannot take changed anvil output from AnvilUpdateEvent

@SubscribeEvent
public void onAnvilUpdate(AnvilUpdateEvent event) {
   if (event.right.getItem() == Items.written_book) {
      NBTTagCompound bookTagCompound = event.right.getTagCompound();
      NBTTagList pagesList = bookTagCompound.getTagList("pages",;
      ItemStack output = event.left.copy();
      NBTTagCompound outputTagCompound = new NBTTagCompound();
      NBTTagList loreList = new NBTTagList();
      for (int i = 0; i < pagesList.tagCount(); ++i) {
         String[] pageSplit = pagesList.getStringTagAt(i).substring(1, pagesList.getStringTagAt(i).length() - 1).replace("\\n", "\n").split("\n");
         for (String line : pageSplit)
            loreList.appendTag(new NBTTagString(line));
      }
      outputTagCompound.setTag("Lore", loreList);
      output.setTagInfo("display", outputTagCompound);
      if (!event.name.trim().isEmpty())
         output.setStackDisplayName(event.name);
      event.output = output;
   }
}

I am using the above code to allow application of written books as lore on items. It works perfectly except for one thing: The result can't be taken out of its slot. Shift clicking and regular left-clicking do not do anything, in fact. Is this a bug or am I doing something wrong?

The reason I am replacing the literal "\n" with the actual linefeed "\n" is that it shows as the literal in the lore. Then I have to split the linefeed "\n" because it renders as an LF icon rather than an actual new line.

Solved by setting event.cost.