On his blog, Frederik Pohl writes about collaborating with Arthur C. Clarke on The Last Theorem (a book I reviewed, asking, "One wonders how stable the manuscript was when Clarke died"). It sounds like an impossible task, and Pohl's blog post is more moving and interesting than the book itself.
For all the time between leaving Chicago and our actual boarding of the riverboat I hadn’t tried to keep in touch with Arthur. But our boat was well provided with email, and one of the first things I emailed him to ask him concerned some interesting alien characters in his notes. These were called the Grand Galactics. They pretty much ran things, and I could see how useful they might be in the finished story, so I invited him to tell me every thought he had ever had concerning these wondrous super-beings. His response, though, was a lot less helpful than I had hoped. Everything he knew about the Grand Galactics, he told me, was in those pages of notes his agent had passed along to me. At one time, he said, no doubt he had possessed any number of additional ideas about them. He didn’t have them any more. They were gone without a trace.
Marveling, worried, I asked Arthur for details. He could give me very few. Apparently a funny thing had happened to Arthur on a day in 2003. After signing all those contracts he had waked up one morning and discovered that he couldn’t remember how to write any of them..
Don’t ask me to explain how this was possible. Arthur himself wasn’t very good at explaining it to me, but there it was. Every word of how to write any of those books had vanished from his mind. He said that since then he had had reasonable luck in writing 300-word greetings to various groups around the world who wanted to honor him. But nothing more ambitious than that. It was as far as his writing skills now went.