Now, go to the Books toolbar item (not the icon thingie), click Books/Create Custom Book Item (or ALT,b,r). Give it a name. Again, click Books/Leave Book Item. (I'm not sure of the importance of this last step, but it doesn't hurt!) 

Now, when you open a book, you should see this new item among the last group on the left, from whence it can be plucked and included as any other item.