Download this tool from github:

https://github.com/NetroScript/Graveyard-Keeper-Savefile-Editor/releases/download/0.1.6/Graveyard.Keeper.Savefile.Editor.-.Windows.-.0.1.6.zip

Extract the archive anywhere and run the .exe
(You need Chrome installed on your machine for it to work)

The program will prompt you to enter the directory for your savefile.
It is located in:
X:\Users\<your username>\AppData\LocalLow\Lazy Bear Games\Graveyard Keeper\

Make sure to have one inventory slot on your character empty, have the game closed and backed up the savefile you wish to edit (usually named "1.dat")

Now, open your savefile.
Scroll down until you see "Add item". Enter "Restoration Tools" (it has autocomplete functionality)


Then, scroll to the bottom and save directly to your saveslot or "export as .dat" into your save directory.

Close the Program and start your game. You should now have the tools in your inventory and be able to give them to the Astrologer, who in turn will then allow you to ask for the Necronomicon.