A specious work written by an unknown individual that both reads and feels like an illusion from the ancient past. It discusses the world avatars inhabit, but seems too absurd to be true. Adventurers note that reading it increases one's summoning magic skill.
Now that you can actually buy them in stacks from the Sparks vendor, instead of one-at-a-time, its pretty nice if you don't have anything else to spend your Sparks on.
Especially for Summoning Magic, which can be a serious pain to skill up the normal way, or if you're just plain impatient.
Depending on what jobs you have at your disposal, farming Sparks is stupidly quick!