I just finished reading Lev Grossman's The Magician King, and it was absolutely wonderful. I can't praise how good this book was. Magic, quests, pop-culture humor, mythology, paganism, snarky humor, and two grand tales.
The book is divided into two complementary parts. One of which is the main conflict and story, the other provides the amazing backstory to one of the characters, Julia, and all the struggles she went through that brought her to how we found her at the end of The Magicians. The stories switch off, chapter by chapter, reaching their conclusions as simultaneously as possible.
What I found most interesting about this structure is that that both stories play out differently to a reader. The mains story plays out like your typical story. It has it's beginning and leads to some unknown end. Julia's story, however, has a foreknown ending. We already know the consequences of what happened to Julia, we just don't know what actually happened to her.
Just as having Julia's story enhances what happens in the main quest, having the main quest enhances Julia's story. I always found myself trying to read between the lines, taking what I knew from one story and applying it to the other. I used what I knew from the main quest to guess at what would happen with Julia, and using what happens to Julia to enhance my understanding of the main quest. The two stories are inseparable, and each would be weaker without the other.
As always, I heartily encourage anyone who is my friend or enjoys fantasy to read these books.