I read this book like 15-18 years ago in middle school.
I think it starts with a banker or businessman finding a coin, I think, and going into a fantasy world, where he meets and falls in love with a plant girl. If memory serves, they had children together.
Sounds like the first book in the Landover series by Terry Brooks - "Magic Kingdom for Sale-SOLD".
A bored businessman sees an advert with this title in the paper; and buys the kingdom. He is transported to Landover, and after many adventures, marries a sylph.
