The file contents aren't all that interesting, but the FBI's Record and Information Dissemination Section's ineptitude and sloth are on full display. I had a request submitted for all their records on Pope John XXIII. They give me the 10 pages of previously released material and then tell me that there might be more records where that came from. In essence, I am told to FOIA again and maybe this time they'll do their jobs.
As for the records themselves, as stated above there's nothing much to them. The file starts with a news article regarding the John XXIII's election. The second document, a three page cable from the American legation in Rome to J. Edgar Hoover regarding the health of John XXIII. In the cable, it speculated that the pope is dying from cancer (he ultimately dies from stomach cancer) and that this knowledge is being concealed by the Vatican due to John XXIII's desire to complete his work. The file concludes with some more press material about Pope John XXIII including a three page article from “Divine Love” a publication based in Fresno California, sent anonymously to J. Edgar Hoover.