…you will probably like The Folk of the Air by Peter S. Beagle.
A man going to visit his old friend has no idea that he is about to walk into the middle of a supernatural war. His friend is involved with a group like the Society for Creative Anachronism but the group has a real witch who can call supernatural creatures from other realms and human warriors from past eras to defend her father’s kingdom.
The visitor quickly discovers that his old friend has a big problem: he’s switching bodies with a Viking who has no idea what is happening to him or why. And the friend’s new girlfriend is something much more than a teenaged witch.
The battle scenes are real and brutal. While not explicit like Game of Thrones, fans of that series of books and the TV show will likely enjoy The Folk of the Air. This book is over 20 years old so you’ll probably have to find it online, but I think you’ll be glad you spent the money.