WASHINGTON — In his first memoir, President Joe Biden reflected on what at the time was the most searing moment of his political career. He had withdrawn from his first presidential campaign in disgrace, driven out by charges of plagiarism. Now, some 37 years after that flameout, Biden’s epitaph weighed heavily as he made surely the hardest political decision of his long career in Washington to give up the presidency after just one term and end his bid for reelection. As he sat in isolation with COVID at his Delaware beach house these past few days, hacking and hacked off,...