For Lennon's 70th, NPR has a great interview with David Sheff:
In August 1980, writer David Sheff flew to New York for a big assignment: an interview with John Lennon and Yoko Ono for the magazine Playboy. Every day for three weeks, Sheff, often with his tape recorder running, talked with the couple in their Central Park West apartment, on the street, in coffee shops and in recording studios. The Playboy interview hit the newsstands on Dec. 6. Two days later, Lennon was shot dead on the street in front of his apartment.
Sheff's interviews with Lennon and Ono were collected in his book All We Are Saying. The book is being re-released as an e-book.
Also, here is John Lennon's Site