But he could just hire a ghostwriter. Hell, thats what he has done on the last 3-4 Westeros history book type things. All he really needs to do is provide the outline, pop in to spice up the dialogue and get that "hey, the guy with the red cup was a reference to this which was mentioned here..." detail, and bam, we are done.
I think he has no real great ending, the series kind of spun out of control, and seeing how his 'outline' was butchered in the show he just gave up. Plus, stuff like the "Hodor" realization was probably like half a book worth of flashbacks, build-up, and rising tension but now everyone knows so it now drags. I bet there will be a posthumous release of everything where he doesn't have to deal with the haters.