I don't think you can get back into the mine, but you can use the
Save Game Editor to give yourself the quest item(s), which will allow you to create the skillbook.
Are you sure none of your characters have the items? Leandra's Blood could be mistaken for a large health potion, though is placed in the Miscellaneous category, rather than Consumables. Leandra's Spell only shows up in the All inventory category; it looks similar to the Reveal Spell (highlighted parchment with an image on it).
The spell doesn't show up in the trade window, so can not be accidentally sold (directly). Last I checked the blood could be sold, though, or (unlike the spell) destroyed (after a conformation window). Either could have been dropped or moved to a container, etc.
In the editor, right click an open inventory slot and select 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. Highlight that, click the 'Select' button and then Apply.
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 result in the items being in the inventory of the character you added them to. You might want to make a copy of your latest save, though, or make an extra save to try this with, just in case.
I tested this awhile ago (in an earlier version of MooseEdit) and got a generic icon for the spell (I also used a random save not up to that part of the story), but other than that it worked fine when read and could be used to craft the skillbook.
