The best I’ve found is this snopes article:

In the book, Unger cites ex-KGB officer Yuri Shvets in making the case that Trump’s relationship with the Russian government started decades before he became president.

…

In a book review about “American Kompromat,” John Sipher, a retired member of the CIA’s clandestine services, concluded that the book doesn’t necessarily reveal anything that was previously unknown about Trump, who has long been a public figure and whose activities have been widely covered for years. It “reminds us that there is still much left to learn,” Sipher wrote.

And the Guardian article about the book has a bit more information about Shvets and his allegations.

So my take is either the information is correct, not dangerous, or the book isn’t popular enough for the interested parties to take it seriously. I don’t know why US intelligence operatives have been silent on the issue, that’s why I hesitate to take the book at face value (maybe they see him as a victim, or perhaps they need a smoking gun).

That’s the best I have. I would really like to hear what the Trump campaign has to say about it. They’ll probably just deflect though.