After one of the longest waits in publishing history — more than 20 years — the third and final volume of William Manchester’s biography of Winston Churchill, “The Last Lion,” is finally about to arrive in bookstores. Manchester, who died in 2004, will not be among those eagerly awaiting its reception. The man with the most at stake is the co-author of record, and in fact the actual author: Paul Reid, who had never written a book before and whose specialty before he met Manchester was features for The Palm Beach Post. The story of how Reid, 63, was plucked from anonymity and thrust into the spotlight is not a simple understudy-replaces-star saga, and it’s safe to say that Reid could not have imagined what a mixed blessing he would experience after accepting Manchester’s invitation to co-write the third volume of Churchill’s biography. Now he has emerged from the project in a kind of literary shell shock, knowing that if the book is a success, most of the praise will go to Manchester, and if it flops, blame will fall on him.

Manchester would have been a hard act to follow for even a much more seasoned writer. Back in the late 1970s, he began his biography of Churchill for what would end up being a $1 million advance. Then a writer in residence at Wesleyan University in Middletown, Conn., Manchester was the author of more than a dozen major works, including “The Death of a President,” the landmark 1967 study of John F. Kennedy’s assassination, and biographies of Gen. Douglas MacArthur (“American Caesar”) and H. L. Mencken (“Disturber of the Peace”). He was also an outsize personality, known for writing sessions that lasted as long as 50 hours and for turning out books at a metronomic clip....