This book is just fantastic. It really does tell the whole story of the behemoth that was Led Zeppelin. The reviews aren't kidding when they say that this is THE definitive Zep biography. I haven't read any of the other bio's, like Hammer of the Gods, but after reading When Giants Walked the Earth I don't want to.
My only criticism is that at some points the italic "in the mind of" sections can be a bit confusing because you don't instantly know who Wall is impersonating.
Overall, this is a must-read. I think I'll be rereading it in a year or two.