The Elegant Universe by Brian Greene is probably the most popular book on it. I would guess that it's fairly easy to read based on both its popularity and the clarity conveyed through the Nova series of the same name. I haven't read the book but I have the Nova dvd's, which I found clear and interesting. I guess the book is a little more technical and detailed.
I'll also recomend Brian Green's novels. I still remember the pop in my brain when I finaly managed to fully grasp relativety. He also paints a pretty clear picture about the what and what-nots of everything quantum, even if you find yourself backtracking.
As far as string theory goes, I really wish they would have called it string model because the patchwork of additions and suppositions leaves this theory hanging by a thread IMO.
Nevertheless I owe a lot to Dr. Green and would recomend his books to anyone interested in physics not just string theory!
Oh and forget the videos, it's like comparing tolkien/potter to their films releases: a lot of shine, but there is just no intimacy.
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