I actually emailed the publisher about this, as the power broker is one of my favorite books (but totally impractical to carry around). I got a response saying

I actually emailed the publisher about this, as the power broker is one of my favorite books (but totally impractical to carry around). I got a response saying that they would love to do an ebook version of TPB, but Robert Caro won't allow them to do that - he even refuses to submit his writing digitally, instead writing everything by hand, then typing it up with a typewriter, which is then retyped by a typist at the publisher into Word.