Searching for a fantasy book title

I'm trying to find the title of an old fantasy book I read at least 20 years ago, but I have very little to help identify it.

Obviously I don't remember the author name, I think it was written in the 80s or late 70s.

It was a classic premise with a modern day man from our world, transported to a fantasy setting. The world I remember was on the brink of an Ice Age and the main character and wizard mentor found a keep to save the human race.

In the keep they found ancient machinery to produce light, warmth and food.

The magic in this world was glyph based, using the true meaning of word to change reality. Obviously the main character becomes a magician even if not really that skilled.

I think there was also a triangle with the main character, a princess and a dead husband who was not so dead after all, but it's all really foggy.

There were at least two books in the series.