(CNN) - In his time as a New York City police officer, Stephen Spiro says that it only happened to him once.
Typically, the people he arrested wanted nothing to do with him.
But then, few things about the case of Mark David Chapman were typical.
Chapman, the man convicted of killing former Beatle John Lennon, wrote Spiro, his arresting officer, four letters.
In the letters, Chapman repeatedly tells Spiro to read "The Catcher in the Rye," saying the book would explain much of what happened the night of the murder, December 8, 1980.
He expresses an ease with the officer, saying he hoped they could be friends, that he thought Spiro was a great policeman, and that he had felt close to him ever since his arrest.
Chillingly, Chapman also wrote that others "could -- and would -- have served the same purpose" as Lennon.
The four letters are expected to go on sale Monday on the Moments in Time website, which sells rare autographs and historical documents.
Spiro says they represent the only time someone he arrested wrote him letters.
"They've been in my possession for 30 years, and I'm 66 years old, and I'm saying, you know, what am I going to do with these things? So I figured I'd sell them," he told CNN.
The letters run from January to May 1983. Each one is typed and includes Chapman's signature.
Spiro says he wrote a few letters in return, but Chapman "cut it off after a while."
The two men spent several hours together immediately after Chapman's arrest. He trusted Spiro.
Chapman, who was denied parole for the seventh time in August, is serving a sentence of 20 years to life.
The letters will be on sale for $75,000, said Gary Zimet, who runs the Moments in Time site.
The site is also selling a "Double Fantasy" record album, which Lennon signed for Chapman before his murder, for $650,000.
Spiro said that he plans to donate some of the money he makes to a local shelter for battered women.
Here are excerpts from Chapman's letters:
January 15, 1983
" ... I am glad that we can keep in touch. I'll not forget the first time we met ...
"Besides wanting to be your friend, there are two reasons for this first letter. First, is there any way that you can help me locate my copy of The Catcher in the Rye that was taken from me on the night of my arrest? Second, in the patrol car on the way to the stationhouse, I remember you telling your partner -- very excitedly -- that you KNEW something big was going to happen to you on that evening. Do you remember this? I would like you to tell me more of this if possible.
"Steven, have you read The Catcher in the Rye yet? I would like you to read it and tell me what you think of it. As you remember, in the copy that was taken from me I had written 'This is my statement'. I am wondering if you now understand this.
"Write soon. Let me know how you're doing. As I've said before, you're probably still the 'best damn cop in New York City'. "
January 28, 1983
"... The reason I wanted to write was from the time of my arrest I have felt close to you ...
"To answer your question of what was meant by 'This is my statement', the only way I can explain is this way: do you remember the young woman in Saigon during the Vietnam war that immolated herself? ... She believed so strongly in her purpose that she chose to end her life rather than to continue living in this phony world ...
"The Cather in the Rye is my statement. The book in incredible ...
"We were all part of something big, Steve. Something phenomenal. It is all going to come out."
March 10, 1983
"... You mentioned your family. Tell me about your children. How about a photo, too?
"Have you read The Catcher in the Rye yet? I know this will help you to further understand ...
"I will let you decide whether Mr. Lennon was a phony or not. His words shoot his life-purpose full of holes. If you dig deep -- and not idolize -- it is all there. Yes, Lennon was a phony of highest degree but there were others who could -- and would -- have served the same purpose. I believe the DA has these names."
May 31, 1983
"Read The Catcher in the Rye.
"Sincerely, Mark David Chapman."
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