You can use the
Save Game Editor to replace the spell (and blood, if you don't have that either).
You may want to double check your characters, backpacks, etc. Leandra's Spell only shows up in the All inventory category; it looks similar to the Reveal Spell, like a highlighted parchment with an image on it (if you were sorting stuff, dropped on the ground it just looks like a parchment). Leandra's Blood could be mistaken for a large health potion, though is placed in the Miscellaneous category, rather than Consumables.
In the editor (a Windows program, but can be run under Wine in OSX), right click an open inventory slot and Add Item, and 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. Leandra's Blood is a few entries above the spell, along with the book (diary). Highlight the spell, click the 'Select' button and then Apply; repeat for the blood, if required.
After hitting the save icon in the editor, loading the save in the game should result in the spell (and possibly blood) being in the inventory of the character you added it (or them) to. You might want to make a copy of your latest save, first, or make an extra save to try this with, just in case.
If you have a problem with that, or don't want to try getting the editor running on a Mac, you can zip your latest (preferably unedited) save and email it to me (raze@larian.com).
