I got this spell thanks to a hero that had it...and it is amazing when using it with enchanted hammers to boost production
I tried some tests to find wich of both was better for a city with only 1 essence, and the result was that Obsession gave more production...(apart from the extra gildar).
And after having workshop and other buildings, the difference was increasing. IDK why, but it is that...
BTW, apart from that hero, is there any other way to get the spell?