William Manchester is most famous for writing the Death of a President (1967). I showed my neighbor a book of his because he is a history buff. He said that he knew of him because his mother cared for him at the psychiatric hospital in Middletown, CT. I would like to know not why, but when he was institutionalized. It seems possible because he was open about his PTSD from World War II, which is reflected in his novel, Goodbye Darkness.
This link says that he fell ill and was in and out of hospitals for a few months in 1989. Afterwords he could not return to his Churchill biography. He would have been about 68 in 1989. Instead he wrote A World Lit Only By Fire, which is a light duty piece of work. He was constantly pressured by fans to finish the third Churchill volume, but in 1998, only had 243 pages of it done. 68 is a little early to stop writing a highly anticipated masterpiece, unless maybe if you're a lifelong alcoholic.
My neighbor only stays here a few days a year. I could track him down in Connecticut to get more details, but I doubt he knows exactly when it was.