The Devil You Know by K.J. Parker
Tradepaperback – Tor.com – Mar2016
The greatest philosopher of all time is offering to sell his soul to the Devil, but the assistant demon assigned to the case has his suspicions, because Saloninus is also the greatest liar, trickster and cheat the world has yet known; the sort of man even the Father of Lies can’t trust. He’s almost certainly up to something; but what?