One does have to question the veracity of these letter, however interesting they are. What do people think ?
Nonetheless someone, or something, has been leaking high level mil-intel to (at least) USA/UK for quite some time. So even if they are fakes, they may have a grain of truth in them.
My partner has a Masters in Psych and is a lawyer. One of the things her agency does is interview and write evaluations to the court for people arrested for domestic violence and some other charges. She is a human lie detector. Her agency does a phone interview with each person, as well as a background investigation and runs a battery of tests. When she does an interview and has a feeling the guy is lying, the deeper dive always turns up what they were lying about. I'm not in her league, but I'm decent at sniffing out when someone is lying.
She's either read the translations herself or I read them to her. We've discussed this very question and both of us had come to the same conclusion. If these are fake, they are written by someone who could write fiction professionally.
This person's analysis of what's going to happen to Russian society is pretty close to my own theories.
A few months ago I saw an interview with a woman who wrote a book on autocrats from Mussolini to the present. She made the point that all autocrat's governments are a hot mess internally. Germany was able to manage around Hitler because the rest of the government was so organized. She also made the point that the longer an autocrat is in power, the less functional the government becomes. Telling the dictator what he wants to hear takes priority over the truth and that is a rot that ends up permeating the government over time.
These letters made a lot of references, in some cases as an aside, how Russia has progressed into this dysfunction over time. Which is congruent with the book on autocrats.
Another thing we've seen in modern culture is that when you have an autocrat (or wanna be autocrat) in a tech savvy country and the autocrat is going off the rails doing things that violate the norms, people high up start leaks. Russian policy for the last 70+ years has been to push the envelope with the west, but don't cross any lines that will start a shooting war with the west. When the USSR realized they went a bit too far putting missiles in Cuba, they backed off.
Most of the people who do the work in the FSB and the military knew that was a massive mistake. Russia's military is just not strong enough to take on a country 1/3 its size (by population).
There are three leaks that I know of. The first was before the war started. Somebody gave the US the entire Putin playbook, probably with the hope that Biden could make Putin back down.
The second is somebody has few the Ukrainians with information about the assassins sent to kill Zelensky. This is probably because they know if Zelensky is killed, he becomes a martyr and the Ukrainians will double their efforts to take out the Russians.
The third is this leaker who appears to be writing a Russian dissident living in exile in France.
Lastly it came out yesterday evening (US time) that the agency of Putin's legal arm swept through the FSB looking for leakers. The head and deputy were arrested. It's unlikely the information claimed to be from within the FSB was fake. Something that became public was traced back to the FSB and it was accurate. It could be about the first two leaks, but it could also be about this person. All three of the leaks could be from the same person too.
I wouldn't say it's beyond reasonable doubt true, but there is enough evidence to have me leaning towards true.