The Summer Tree, by Guy Gavriel Kay, is the first book in The Fionavar Tapestry series. This is a YA/Adult book. This book is not for children. That being said, if you are old enough to read this book, I found it to be complex and interesting.
The story begins in our world, in Toronto, Canada. Five college students are lured into traveling to another world, Fionavar, by a man, Silvercloak, from that world. He is a mage and can do magic through his source, Matt, a dwarf. Mages in Fionavar have no power to do magic without their source.
Once in Fionavar, the five college students each learn about Fionavar through separate experiences. Fionavar has a long history of good and evil, gods, goddesses, and kings. The evilest of all the evil in all of the universe was bound beneath a mountain ages ago in Fionavar. This story is about the events leading up to the release of this evil being and how the five college students from Toronto got involved in the saga.
Guy Gavriel Kay's writing style is detailed and rich. His story is epic. I recommend this book to young adults or adults that enjoy detailed fantasy fiction.