Riddle of the Seven Realms by Lyndon Hardy

"Riddle of the Seven Realms" is an excellent follow-threw of the trilogy. There is Kestral, a conman who tricks wizards into buying what they think is Anvilwood when it is really a hump of worthless wood; Pheobe the lady wizard who Kestral tries to trick; and lastly Astron, the one last walks, who is a demon, a sibling of mighty djinns, who follows Prince Elizar, the one who is golden. A twist of fate brings them together. When Phoebe insists on testing the anvilwood smaple that Kestral brought, she summons forth Astron, who dominates the wizard. Elizar had been attcked by a second demon prince, and now the fate of the demon world hung in the balance as the other demon prince posed a riddle: How does one light a fire in the realm of Daemon (demon)? Astron searches for the answer in the realm of men, seeking out to Archimage Alador, from the first book, "Master of the Five Magics". Then they continue on to other realms: the realm of the fey, where they meet up with a beautiful hillsovreign, and then realm of symmetry and others. At last they return to the realm of Daemon with what they need to find out the answer to the riddle: but logical Astron has already discovered it and now knows, from his experience in the different realms, that the answer will surely destroy them all.