Abraham Pais was a also physicist, a younger protege of Einstein's at the Institute of Advanced Studies at Princeton. He knew Einstein personally; he was versed in the details of his work; he was uniquely qualified to write an intimate biography. His book is remarkable for its level of detail. Book publishers claim that adding a single equation, even one as "simple" and well known to the general public as , would reduce the readership by half.
If that's true, and knowing how rapidly converges to zero, then I must have been the only person on earth who ever bought and read Pais' work. Not only did he have plenty of equations, they included generalized tensors with sub-scripts and super-scripts proudly waving. I'm neither a mathematician nor a physicists, but I've taken a course that included generalized tensors. The presence of metric tensors didn't scare me away or detract from my enjoyment.
As much as I liked "Subtle", I think Isaacson's book was more fun and accessible. He made Einstein seem less like an intellectual, unapproachable deity and more like a real human being. I could appreciate him expressing his frustration towards the folks at Zurich Polytechnic for dithering about a job offer by saying on page 176: "The dear Zurich folks can kiss my..."
Einstein was a man of great humor. He was quite a hit with the ladies - divorced once, married twice, and amorous with a few more. He loved music, especially Mozart for his violin. He was detached from people, yet he had friendships that lasted his entire life.
Walter Isaacson is a terrific writer. I enjoyed this book very much. It makes me want to grad his Steve Jobs biography as soon as possible.