The problem, German compellingly argues, isn’t simply that the F.B.I. has targeted innocent people in the name of national security. It’s that the agency’s obsessions fuel discrimination and violence against racial, ethnic, religious and political minorities, while allowing other security threats — like those now posed by white nationalists — to go unchecked. Consider what happened after 9/11: Resources were diverted from domestic extremist terrorism and into investigations of Islamic terrorism, even though statistically, homegrown right-wing extremists are arguably a greater threat.

Other large-scale crimes were also neglected, German charges. For instance, under the director Robert Mueller, the F.B.I. “transferred about 500 agents from white-collar crime squads to terrorism,” leaving only “a skeleton crew … to address the growing mortgage fraud crisis.” Then, after the 2008 financial crisis, the F.B.I. “failed to hold the top corporate beneficiaries of those frauds … liable.”

At times, “Disrupt, Discredit, and Divide” feels more like a diatribe than a work of careful scholarship: German gives readers little context against which to evaluate the F.B.I. as a whole. He doesn’t say much about its successes, making it hard to know whether the flaws he cites are the exception or the rule. But this, German contends, is part of the problem: Since there is little public data “on how the F.B.I. uses its intelligence and investigative powers,” it’s almost impossible to evaluate whether the F.B.I. gets it mostly right or mostly wrong.

Still, German’s book is ultimately a powerful argument against idealizing the F.B.I. and overlooking its troubling record for the sake of short-term political convenience. In today’s divided America, the enemies of our enemies are our friends; if Donald Trump thinks the F.B.I. is full of “corrupt,” treasonous “losers” on a “witch hunt” into his connections with Putin’s Russia, Trump’s critics are quick to praise the bureau for championing the rule of law. But while Trump’s attacks may be unfounded, the F.B.I.’s many failings over the years have contributed substantially to public cynicism, German contends — and “when people lose respect for government institutions and confidence in the rule of law,” the result is predictable: They “look for stability in authoritarian leadership, placing their faith in a strong individual who can ‘drain the swamp.’”