J. Michael Lennon, a mild-mannered retired English professor, will forever be linked to the combative Norman Mailer. His 900-page biography, “Norman Mailer: A Double Life” (Simon & Schuster), was published this past fall to wide acclaim, and Lennon is now in the throes of editing 50,000 of Mailer’s letters for a book to be published in the fall of 2014. It was a letter, after all, that first joined Lennon’s life to Mailer’s.