I'm not a professional Grant scholar, but I have read 12 books (and counting!) about Grant so far, so perhaps you'd like to hear a few things I've learned that I can recall just off the top of my head:
1. For starters, Twain was not Grant's editor; he was Grant's
publisher. There's a big difference. Grant was pretty much
his own editor.
2. Adam Badeau had written his own book some years before, but when he came to help Grant with Grant's memoir, he was only a research assistant. Grant's memory was remarkable -- we have testimonies from numerous people who knew him throughout his entire military and presidential career, and one thing they all say is that whenever you got Grant talking, he was a wonderful storyteller with a prodigious gift of recall -- but even Grant could not recall every single date. He was scrupulously honest his whole life through as well, and did not want any careless errors to be propagated, so he had his son Fred, as well as Adam Badeau, on hand to check records and make sure that all the facts regarding battles, etc., were correct.
3. Grant refused morphine as much as he could, because he did fear that it would cloud or slow his thinking. The standard way to relieve the severe throat pain then was to numb his throat with cocaine, and this his doctors did. Dosages of cocaine, morphine or anything else were probably pretty fine-tuned, since Grant kept extremely detailed notes for his doctors, hoping that the information would be helpful in the future for doctors treating others in his condition.
4. Grant both wrote and dictated (to a stenographer) for as long as his voice would allow, then he went solely to writing. It is hard for most of us to conceive of doing that kind of work when one is in such pain and discomfort. He could not even sleep lying down, because the tumor would make him feel like he was choking; he had to sleep sitting in a chair. But Grant was an amazingly tough individual. When you read his life story -- and we have details from people who saw him day in, day out, for years at a time -- you are struck with his incredible tolerance for pain and discomfort. He suffered from horrible migraines, and he soldiered on. He broke his leg and had to be lifted onto his horse, with his crutches strapped on beside him for when he got to his destination, and he soldiered on. When he set his mind on something, he simply did not let anything stop him, not even excruciating pain. I know I couldn't write a book under such circumstances -- but I'm not Ulysses Grant.
If you really want to know what Grant's final year was like, there's a whole book about it:
Grant's Final Victory: Ulysses S. Grant's Heroic Last Year