Greetings Newbie!
Welcome. Here are some tips on the letters. I have written extensively on them, and have a full chapter on them in my book at Amazon - ten bucks: DB Cooper and the FBI - A Case Study of America's Only Unsolved Skyjacking.
You can also go to the Mountain News. I have about 60 pieces on DB C posted there. Google Mountain News and DB Cooper.
I don't think the letters are from the skyjacker, but others think so, including the enigmatic "Al DI." Who his guy is a mystery, and could be an FBI agent playing games or trying to ferret information out of his agency.
The letters certainly reveal other kinds of mysteries that tell us about the nature and scope of the investigation. The letters are generally lumped into two categories. One are the pithy, haiku-like letters, and these are the ones that attract the most attention. They are the most enticing and sublime. All the other letters are from folks who are easy to dismiss. They are egotistical and poorly written.
The last of the Core Four was sent to the Oregonian in the week after the skyjacking. It was a paste-up job using letters from the June and July 1970 Playboy. Al Di has "decoded" them. Interesting analysis. This fourth letter was not published by the Oregonian, but was sent to them, who forwarded it to the FBI in Seattle. Astonishingly, the letter was posted the next day in the Billings Gazette, in Montana, presumably to smoke out any accomplices Paul Cini might have had in the area, as Cini's skyjacking touched down in Great Falls on November 11, 1971.
This was before fax machines and the such, so the FBI had to photograph the letter and then courier it by plane down to Billings. All in less than 24-hours, too, so the FBI clearly took this letter seriously, at least as a ruse for any Cini compatriots.
Many in the FBI consider Paul Cini as the first skyjacker (two weeks before Cooper's) who was ready to parachute away, but I consider Dan Cooper to be the originator of the classic 727 getaway, as Cini was using a DC-8 and it only had a side hatch, not an aft stairway.
I have also written extensively on these "copy cats," and discuss if there might have been a link between them.