Reviews of the book back in '74 pointed out that the author (former CIA) had had the CIA vet the book before publishing, and so the reviewers didn't trust it 100%--there might be things he covered up.
True; read it with that caveat. But it described a far more plausible way to use blackmail material than the "get this info for us or we tell the world about your embezzlement" MO that comes to mind first.
It depended on the target, of course--some personalities wouldn't be malleable enough--and it had more moving parts, but it seemed to this amateur that the method would more likely be fruitful than not.
If you really have the goods on somebody and you try to get them to do something else unethical, and maybe dangerous, you run the risk of a "publish and be damned!" reaction, or suicide. Neither helps you, the spy.
The setup he described worked a little differently.
The agent gets to know the target casually, and builds a relationship from there. When the time seems ripe, a third party threatens or warns the target that he has incriminating evidence. The agent, allegedly hearing of this through some plausible mechanism, offers to help--somebody owes him a favor and he can call it in, no questions asked. The agent "helps," the incriminating evidence is given to the target, and it is never mentioned again.
But the target now owes the agent big-time. He won't be as averse to doing the agent a small--almost trivial--favor. If the agent carefully calibrates the increasing favors, and the increasing rewards that go with them, the target can grow into a nice reliable source via a "gentle slope, soft underfoot, without sudden turnings, without milestones, without signposts."