Here's what I found most interesting in skimming through it:
1. There's a chronological narrative by a Secret Service Intelligence specialist, A.D. Kelley, beginning on page 20 of the pdf file and running through to the end (p. 60). Entitled "Secret Service Participation in Tapings," the report is contemporaneous -- it's dated July 2, 1974, only a few months after the incidents and actions it describes -- and it's authoritative both in the sense that it comes from operatives who were themselves participants in the events described and in the sense that it comes from an organization that wished to preserve its integrity and reputation for rectitude.
2. It details the installation of taping equipment in the White House, Exec Office Building, and Camp David; and it explains how the equipment worked, how the tapes were stored, how transcribed, and who had access.
3. There's extensive coverage of Secret Service involvement, including a statement of its reservations about getting involved in the taping program in the first place.
4. Kelley gives a first-person account of the notorious 18-minute gap in one tape and how it resulted from a process of erasure and forgery by Nioxon's personal secretary. This episode, Kelley says, produced a temporary embarrassment in the Secret Service until it was learned, he says, that their agents were not involved (initials on the control documents were forged to make it appear that they had control when they actually did not).
There's a lot more of course, along with a great deal of redaction.
Reading it brings to mind the turmoil of that time, and how damaging it was to my own view of the integrity of government -- coming as it did on top of many other damaging revelations (the trumped up Gulf of Tonkin incident, to take one random example).
At that time I had a friend who worked as a staffer on the congressional committee conducting the impeachment investigation. She actually sat all day listening to the tapes, transcribing them, and comparing notes with other staffers and the committee counsel on their significance to the impeachment. Quite exciting.