The story is about a successful business man named Ben(?) who is very bored of his life.
He finds a old man - trader of rare things who offers him an amulet that serves as a key to the gate to another world where he would be a king.
After he agrees and the deal is done he travels to that other world, where he is nearly killed by a chasing shapeless monster in the "corridor". He finds himself in a beautiful land with a once prosperous kingdom, brought to its knees after the old king's death. The trader of rare things makes his living by selling this artifact to people of Earth hoping that they will run from the world as soon as they find out about the duel they have to fight.
The amulet has a secret power to summon a knight who is the king's own entity, although the bearer of the amulet cannot really control himself while the knight is summoned.
Here are a few memorable moments that might help to remember the books:
The king has a few loyal servants: a mage, who is really bad at magic. A huge, talking, bipedal dog - a former human, transformed by the mage who cannot return to his true form. A green woman (Silvida?) who the king marries while on a quest to unite the kingdom.
If somebody could help me to remember the name of these books, I'd appreciate it a lot!