I read a book in my high school library back in the late 90s and pretty sure it was written in the mid 90s. I can't remember the author or title.
The setting is current to the 90s mid-west. But it also takes place in an alternate medieval style fantasy world that only certain individuals can go to via falling asleep.
The main character is a gas station attendant in his early 30s, whose father left or died and he inherited a storage container full of odd stuff. For as long as he can remember he has had these real dreams where he is a monk at a monastery scribing books. He has no knowledge the dreams are actually a real world, that he's part of a family who can travel between the worlds.
Those that can travel between the worlds also possess binary magic, it uses opposites: life/death, fire/water, strength/weakness etc. The main character discovers that he can use life/death. Two strangers save him from the monastery, a woman using strength/weakness and her son who uses fire/water.
The two strangers who rescue him from the monastery are trying to take him to their family holdings to explain why he is important and what the other world is.
In the modern world he is falsely kidnapped and rescued to introduce him to the families, because they want a magical relic that each family has and that his father has taken and hidden.
Because he is new to binary magic and not taught the same rules as everybody else growing up he learns new things that were never considered, such as holding the bond open so he can see other people using magic etc.