The Summer Tree is the first book in The Fionavar Tapestry series (continuing with The Wandering Fire and The Darkest Road).
At Loren Silvercloak the mage's bidding, Kevin, Kim, Jennifer, Paul and Dave, five University students from Toronto, 'cross' into the Kingdom of Fionavar, the First of all Worlds, to help him in an oncoming war against Rakoth the Unraveller, and thus fulfil their destinies.
Like the five heroes, the reader is teleported into a land of magic and fantasy which the author only describes little by little. Although certainly used as a stylistic device, this sometimes makes it hard to understand the ins and outs of certain characters' actions. In the same vein, I also found the few sex scenes somewhat a bit out of place and unpoetic. Anyway, looking back upon it, I realize these were just details.
And indeed, as you turn the pages and learn more about the people and history of Fionavar and about the role the heroes have to play in it, the book really turns out to be enthralling and hard to put down. I particularly enjoyed the third part, where Dave is taken in by a tribe of hunters called the Dalrei, and learns about their customs and rites, to finally risk his own life for them.
Although at first I was a bit sceptical about the mixing of today's world with fantasy, in the end I really liked this book a lot and I'm looking forward to reading the next two. So don't let the first 150 pages or so get you down and read on, it's definitely worth it!