Who is Duncombe from Tales of Henry James: The Texts of the Stories, the Author on His Craft, Background and Criticism and what is their importance?

Duncombe is the author of the novel The Middle Years. He is past middle age, and convalescing from an unspecified serious illness. Duncombe was married, but his wife and son are dead. He feels that he has just reached his full powers as a writer, and his greatest wish is to have a second chance at life, to write more books. Unfortunately, his health deteriorates and his wish is not to come true.