The overarching principle of elvish morality is summed up in the popular form of Thorwald's Edict:
"ELVES DO NOT LIE."
Scrupulous truth-telling had been an elvish tradition dating back to the Long Ago, untold centuries before Thorwald made it the official law of the Empire. Part of an elf's magickal nature requires that it not tell lies. To utter a falsehood is to undermine the integrity that is the foundation of the elf's soul. When he lies, he compromises his very existence as well as that of Faerie itself. Elves are mystically and inextricably linked with the Shining Land itself, so each lie taints not only the individual, but all elves collectively, as well as the fabric of Faerie.
Note that although elves are forbidden to tell a lie, they are in no way obligated to tell the truth.
The law allows certain limited exemptions for "fiction" as long as it is designed purely for entertainment purposes and is not deliberately deceptive.