Are you sure none of your characters have the spell? It only shows up in the All inventory category, and looks similar to the Reveal Spell (highlighted parchment with an image on it). You may have dropped the spell, or put it into a container, etc.
You can use the
Save Game Editor to replace the spell, which will allow you to create the Death Bane skillbook. I tested that awhile ago (in an earlier version of MooseEdit) and got a generic icon for the spell, but other than that it worked fine when read and could be used to craft the skillbook.
In the editor, right click an open inventory slot for a character and Add Item, then under Base Item, Objects, Generic, near the bottom, just above a section of different book series, is 'Activated Control Panel', with a list of items underneath, which should include Quest_LeandraSpell. Highlight that, click the 'Select' button and then Apply.
You don't need them, but Leandra's Blood is a few entries above the spell, along with the book (diary).
After hitting the save icon in the editor, loading the save in the game should replace the spell. You might want to make a copy of your latest save, though, or make an extra save to try this with, just in case.
