You have two options that don't involve messing with game files.
One is just to wait, and make do without the particularly spells you're wanting until the vendors have them to buy.
The other is to craft the skillbook if you have the necessary scrolls (which you can, possibly, get by crafting). You can find a recipe on the forums here.
Also
There is a magical poison arrow skillbook by the lighthouse in the western section of Cyseal if you want an offensive Geomancer spell