For me this is a one star read, but I feel guilty about the first rating for a book being one star, especially if it might be due to my expectations for the content. Once some other people have reviewed it I will re-add my rating.
Most important recommendation: If you are going to read this book, choose physical or ebook over audiobook. As the book is less a commentary on this MI5 surveillance, as I had initially assumed, and instead more of a Cliff Notes of what actual documents are available, there is a lot of reading out of alphanumeric codes to identify individual records, which you will not remember if you want to actually go and look up the records in question.
And actually wanting to look up the records in question is probably the main reason you would want this.
The closest comparison I can think of is - in 2010, Wikileaks dumped huge numbers of diplomatic cables, which were mostly a slog to trudge through, and so in light of that people who had already done some of that work would highlight, organise, and quote specific cables you might be interested in reading on a certain topic. This has a similar energy, just a 15 hour recital of lists of people who were being surveilled and a couple of comments on what they did or what MI5 wrote about them.
Not recommended.