Sometime in the early 90's, I borrowed an audio book from a local library. I would be interested in reading it in full, but I don't remember the author or title. Given that it was an audio book in a small town library, I suspect the novel was originally written in the 70's or 80's.
The basic premise was that there were many different worlds, but all of them - including our own - were just "shadows of the one true world."
In the first scene, the protagonist (a middle-aged man) is visited by an old childhood friend. The friend reveals that he is a wizard and asks the protagonist to come with him to the one true world.
In a later scene, the two characters are riding in a truck, but when the wizard's spell takes effect, the scenery changes from a highway to a dirt road in the middle of a forest. Likewise, the truck transforms into a carriage.
I didn't get much farther than that and the rest is hazy. I vaguely remember the description of a city filled with ivory towers, nestled in forested hills and alongside a river or lake. Also, if I recall correctly, the friend was forced to go, leaving the protagonist alone in the city; other wizards were scheming to use the protagonist to further their own ends.
Any ideas of what the novel may have been?