Divine By Choice by P.C. CAST

Shannon Parker has finally come to terms with life in the mythical world of Partholon. She loves her centaur husband, her connection to the goddess Epona and the pampering that comes with both! She's almost forgotten her old life on Earth– especially when she discovers she's pregnant...

Then a sudden burst of power sends her back to Oklahoma. Without magic, Shannon can't return to Partholon– and so she needs to find help. Trouble is, it might take the form of a man as tempting as her husband. And along the way she'll discover that being divine by mistake is a lot easier than being divine by choice