Apollo 8 by Jeffrey Kluger is one of my favourite books. It was basically extraordinary that NASA gambled so extravagantly, combining multiple steps in the planned sequence into one mega mission in order to get back on schedule after the disastrous fire and meet Kennedy's aspirational goal. Chances of success? (surviving the mission), maybe 50%. Thinking about Frank Borman being alive and well and still flying his pl…
"When Shepard was grounded in October 1963, Grissom and Borman became the prime crew of Gemini 3.[41] Grissom invited Borman to his house to talk to him about the mission, and after a long discussion, decided that he could not work with Borman.[34] According to Gene Cernan, "the egos of Grissom and Borman were too big to fit into a single spacecraft".[44] Slayton therefore replaced Borman with John Young.[41]
Slayton still wanted Borman for the two-week flight [...] so Borman was assigned as backup commander of Gemini 4, with Jim Lovell as his co-pilot. [...] Prime and backup crews trained for the mission together, and Borman found the experience as a backup valuable, amounting to a dress rehearsal of their own mission.[49]