SpaceX would probably be fine with Gwynne Shotwell running it, absent Musk, at this point. Perhaps they'd lose some of their media/PR attention and the daring to take chances that Musk is known for. I'd expect SpaceX would become a bit more conservative in its ambitions, and would probably continue to operate well and be very successful as a business.

It's the Apple model without Steve Jobs. Cook is a tremendous operator, he's just not a visionary or new product guy. Apple won't have to worry until the next big inflection point. SpaceX could probably ride - with modest incremental improvement - on its technology lead for the next 10-15 years.

In the case of Tesla, the best thing they could do is hire a talented operations heavy CEO and have Musk be the product lead. He has never shown himself to be a particularly talented CEO, it'd be better if he focused on what he's the best at.

The book by Ashlee Vance is the authorized biography which Musk helped write.