The book is essentially a dossier compiled by Garland Briggs. It is read and annotated by Special Agent Tamara Preston at Godron's request.
The jade ring we see in FWWM is first seen as far back as Lewis and Clark's expedition west.
Owl Cave has a history of sightings, described as a 6 foot tall "owl/man."
There is a mention of the nuclear blast at White Sands.
The main character of the book, if there really is one, is arguably Doug Milford. He was the most minor of characters on the show, as the old man who runs the newspaper who marries the beauty queen and then dies. Just a throwaway gag originally, but expanded here to be a shadowy character of great importance behind the scenes. The book follows his involvement in all kinds of conspiracies across the 20th century.
He first sees what he describes as "Bigfoot" back in 1927. Then he's a witness to the Roswell crash in 1947. At some point he is abducted himself, and bears the "triangle tattoo" that other abductees have. He becomes one of the Men In Black, may have even had something to do with Kennedy's assassination, and at one point is hanging out with Richard Nixon and Jackie Gleason as they go to a top secret facility to see an actual alien. (It is implied that Nixon himself was in possession of the jade ring at this point.)
The alien they see is described in a manner that makes it sound very similar to the thing we see in the "box" in New York that kills the couple in the first few episodes of the new series.
It is revealed that Andrew Packard and Pete Martell died in the bank vault blast. Audrey was critically injured, but survived. There was speculation that Pete shielded her from the blast with his own body. Audrey was left in a coma.
Three children went missing and had an abduction experience in 1947. They were Carl Rodd. the Log Lady, and a third boy named Alan Traherne. He apparently died of cancer in 1988, before the Laura Palmer case took place. But we have seen Carl since then, both in FWWM and in the new show, where he saw the dead boy's "soul" leave his body. Log Lady also looked to be dying of cancer, but that might not have been intentional as the actress really did die of cancer right after filming.
All three abducted children also had the triangle scar/tattoos on their body, same as Milford and later Briggs.
The first mention of Harry Truman having an older brother Frank was in detailing their high school football days, when most of the Bookhouse Boys were on the team together, including Harry, Frank, Big Ed, Jerry Horne, and Hank Jennings.
Hank Jennings was fatally stabbed in prison, supposedly by a distant Renault cousin.
Milford returned to Twin Peaks after "retiring" from the Air Force, and he buys the town newspaper. But really, he establishes Listening Post Alpha in Twin Peaks. LPA is a SETI facility that is trying to monitor alien life. Briggs is chosen as his successor, and he gives Briggs the documents that becomes the dossier that makes up the book.
It's mentioned that Leland Palmer was born in 1944, so perhaps that puts him at the right age to be the boy we saw walking with his girlfriend in episode 8? Maybe not.
Phillip Jeffries and Windom Earle are mentioned very briefly. Agent Chet Desmond is also mentioned in the fact that he disappeared during the Teresa Banks investigation and was never heard from again. His partner on that assignment, Sam Stanley, apparently had a breakdown shortly afterwards and never came back to active duty.
The theory is thrown out that maybe Milford's young bride was actually as assassin charged with silencing him once and for all?
Once Milford is dead, Briggs needs a new partner for his SETI project, and he thinks Cooper is perfect. Cooper comes to see him after waking up at the Great Northern. Briggs knows something is terribly wrong. He writes "mayday" and heads to Listening Post Alpha alone and that's the last entry.
We know from the show now that Bad Coop sets fire to LPA and Briggs dies while inside. Of course, there's more to it than that, cause of his body turning up 25 years later and his head floating around in the Black Lodge.
But that's pretty much it....the Secret History of Twin Peaks.