ICOT: It wasn’t that tricky, and you guessed correctly, though not for the right reason. I just wrote “Senator” Paul Simon because people always confuse the two. Walt Whitman (died 1892) did not write a biography of Lincoln, though he did write many Lincoln poems after his assasination. “O Captain! My Captain!” may be the most famous, but “When Lilacs Last in the Door Yard Bloom’d” is more typical of Whitman’s style.
Dale Carnegie wrote _Lincoln the Unknown_, the poet Carl Sandburg wrote a biography for children as well as a multivolume biography for adults. Paul Simon wrote a number of historical books, so it’s not surprising that he wrote one about Lincoln, given that both gained national prominence while working in the same city.
If you’re interested you can find the text of “All Summer in a Day” online.