I have discovered a needful book, or at least I find myself wanting a book which does not, to my knowledge, exist. Plenty of Neil Gaiman exposure of late has me thinking of the popular literary device of gaming, gambling, and bargaining in a mythic perspective. Such battles of wit and will often carry implicit, or even explicit references to The Rules, which one breaks at risk to one's own soul, or at least the loss of the challenge. The tales build upon their own tradition; reusing story elements suggests some degree of consistency, that the games continue to use the same Rules, possibly refined over the ages to close up the loopholes which make for such good stories.
Let me give you a few examples.
Ever heard of the wish game? A magical creature grants a wish or three to some hapless mortal whose objective is to get the most from the wishes, while the spirit tries to pervert the wish to the mortal's detriment. King Midas played and lost. So did the fairy tale couple who wound up with a sausage attached to the husband's nose. The Monkey's Paw is a more recent retelling which openly acknowledges the malevolence of the wish-granter.
A variation is selling one's soul to the devil. Sometimes, the soul is lost, a la the original incarnation of Faustus. Often, however, the story reads better when one outwits Satan, as in The Devil and Daniel Webster or Convergent Series. The sinner wins by exploiting a loophole in The Rules.
There's the Achilles' Heel monster. Originally, a vampire could only be dispatched by impaling its heart with a stake of beech or lightning-felled oak, or by decapitation followed by stuffing its mouth with communion wafers. More recently, movie vampires have developed more vulnerabilities; now it suffices to trick the creature into remaining out until dawn, or luring it into prolonged contact with a cross. Still, the game remains the same: dispatch an otherwise invulernable creature by tricking it into its one weakness. Gandalf used the same tactic against trolls early in The Hobbit, and Superman continues to banish Mr. Mxyzptlk to the fifth dimension by getting the imp to say his own name backwards.
Oath-taking is sacred, and was once taken quite seriously. In the context of a mythic contest, they become Rules. One of The Rules is that participants can make their own before they start.
Riddle contests are universal.
The list is long. The Greek Furies can't touch their victim; they can only goad their victim to suicide. Similarly, vampires can't come in until you invite them. Breaking bread together is a sacred bond. Violating the trust of a house guest carries horrific penalties, and not even the Aesir can kill a blood relative. Holy ground is proof against all manner of evil, not just Christian bugaboos. Djinni are helpless sealed in a bottle. So are leprechauns once held by the leg. These are Rules.
Susan Cooper's The Dark is Rising series describes several tactics in the battle between the Light and the Dark, hinting that we're only seeing a small sample of the Rules, that somewhere is a vast compendium of all the opportunities and restrictions. If we could get our hands on it, we could describe every fairy-tale battle ever fought, or that ever will be.
That's what I want to see: an encyclopedia of The Rules, culled from myth and legend and fairy tale. It wouldn't be self-consistent, alas, there are too many contradictory versions of the stories floating around. But it would be one hell of a read.