Hello JoeS1 and welcome to the forum!
While I agree that more book reviews would be welcome, there are many things in your posts I rather disagree with, I hope this post doesn't come off as pretentious or the like.
The community isn't always very friendly or open about discussing Go. There are typically several hundred people on KGS yet there is almost never any discussion about Go or high level Go. No Dan level players discussing how to help people around the kyu level. You typically only ever see people around 5kyu and weaker ever talk. I've asked questions and seen questions asked, but even though there are 500 people online only 1 or 2 talk. So if you want to ask dan level players for their thoughts on improving or books to get at your level, you get no answer.
Well, I don't play too much on KGS but when I do I don't spend my time looking at the discussions in various rooms, so yes I guess questions would largely go unnoticed there. On the other hand, several ddk/sdk players have asked me things in private (teaching games, questions about one of their games, etc) and I always took time to play/comment/answer them.
Lack of knowledge on the subject in the West would be another one. A lot of people have to resort to books to learn unless they can afford to get lessons from Pros regularly. While there are plenty of books around the beginner level, once you start to approach shodan level there become fewer and fewer books to get that help explore and master the basics. That's what I think is key books that spend the entire book focusing on a concept, like aji, or sabaki, or invasion, etc. So that you can learn and master it. Not like the beginner books that only spend a chapter and a few examples on it. I feel like I have major weaknesses when it comes to the fundamentals that could be addressed but no one, even when I've played pros have been able to tell me what my weaknesses are and what I should work on. They just tell you 1-2 mistakes you make and that's it. Kind of difficult to grow from that.
I can only speak from my experience but, having played hundreds (thousands?) of handicap games as white, it is my opinion that not commenting about too many things is more efficient than commenting about a lot of moves. I now make sure to hammer in one or two ideas/sequences per game. And even then, it takes people a long time to integrate these in their games. Repetition is key here, more than diversity I think.
People are less likely to quit if they have a direction to go in. The more dans in the community the more competition there will be. The more high level discussion there will be as well. There is also a higher likelihood that they can set up clubs where they'll be confident to teach. This seems like simple stuff that could help grow the community, but hasn't been done at all over the past several years as I've played Go.
In my experience people don't come (and stay) to a go club because of the competition but because of the friendly atmosphere. Sure, discussion about the technical aspects of the game (wether high-level or not) is necessary but I would not say it is the primary reason people come.
About the book stuff. I know some of the older books from the 60s and 70s were high level, but I was talking about more modern (past 10-15 years) books as Go has changed a lot.
Not sure that go has changed that much over the last decades. And as far as we, weak amateurs, are concerned I don't think there's anything to be learned about go that cannot be learned from older books.
The books don't have to be dan level. They could be sort of intermediate level around 3-5 kyu KGS minimum and stronger. They could focus on each concept and fundamental and helping players understand and master it. As far as I know the only Dan book level series out there is Graded Go Problems for Dans,
But this kind of material already exists. How would you rate a book like Attack and Defense? It is one of my favorite books and I don't feel like I'll ever be able to master its contents entirely. I don't think the divide between kyu/dan material is a really valid or useful one. There so many things dan players do not know or master, even things found in books that are supposedly aimed at lower levels.
It's kind of an expensive hobby as there is so much knowledge and so many concepts out there on it.
To me, this is related to the point above. I don't think there are so much concepts to be learned or so many books to be bought. The problem, as I see it, is more about understanding (deceptively) simple concepts in a finer way as one improves. Ideas/concepts like "urgent before big"/"don't play close to thickness" are simple and a 20k can understand the basic idea behind them. But to apply them in your games and understanding what really is "big", "urgent" and "thickness" is a never ending quest and I don't think this can be helped by buying plenty of books. Here also, I think repetition is key, one good book (or chapter of a book) and plenty of games played/commented could be enough.
Another idea I'll throw out there that I had for a book.
Have pros play amateurs around 3-5 dan at least KGS in even games. Maybe even a few around 1 dan KGS. But have Pros play amateurs in even games so normal josekis can be played. About 50-100 games. Then have them review the games, publish the more interesting ones. The reason I said probably 50-100 isn't to publish them all, but to see if there are common mistakes amateurs make that they don't know, that pros can correct. I don't expect the games will all be interesting but some of the most interesting ones could be published. That might be a popular book if you get some top pros to do it as sort of a charitable thing.
Why dismiss handicap games?