Leatherhead
Possibly a Idiot.
5th generation sounds about right, Genasi live for up to 150 years. Assuming ~30 year generations, that would mean the human ancestor lived some time around the Time of Troubles, which was a wild time to be alive for sure.Actually, I like the idea of her being your great (great great...) grandmother, and the line has been Genasi since she loved a human (long gone, I'm afraid). Your pact would be family loyalty - you are the one entrusted with the family's book of shadows (though I think "shadows" is wrong in this case - it should be something else.)
As for the book, maybe it's a "Book of Wonders?"