Where can I find the story of Magic the Gathering?
I don't read them myself, but every year Wizards of the Coast releases novels which are a companion to the latest Magic sets/block.  If you want the complete "story behind the game", this is probably the best path to go down.

Here's a useful link: http://en.wikipedia.org/wiki/List_of_Magic:_The_Gathering_novels

The Magic Daily site also has quite a fun column called "Savor the Flavor" which contains essays on the latest sets from a storytelling and flavour perspective - you can't get a full picture of what's going on from reading it, I don't think, but it may appeal to someone who is interested in the ongoing story aspect of Magic, not just the mechanics of the game: 

http://www.wizards.com/magic/magazine/Archive.aspx?tag=SavortheFlavor&amp;description=Taste%20the%20Magic