In a later meeting to discuss the forceful restructuring that Mr. Summers proposed — firing management, unloading assets, possibly forcing banks’ creditors to take losses — Mr. Geithner describes his response this way: “Sure that might make sense in some cases. We’ll see. We couldn’t know what would be optimal months in advance; it would depend on the bank and the state of the world at the time. … We were also constrained by the limits of our authority, which didn’t really allow pre-emptive nationalization. We still had no way to wind down complicated financial firms. … We were also constrained by the limits of our remaining financial resources.”

In other words, a more radical approach to the banks might make sense in some first-principles, what-is-fair sense of the term. It just wouldn’t work within the practical constraints that he was grappling with in trying to actually execute the policy. He argued that the stress tests he engineered, carried out by the Federal Reserve using existing legal authority, would be tough enough to restore the banking system to health, even if they lacked the visceral punch of firing C.E.O.'s and nationalizing banks.

You see something similar in the financial reform legislation that would become the Dodd-Frank Act, which Mr. Geithner was a guiding force in designing. There is no question the law brings greater oversight to the biggest banks. But it also doesn’t reimagine the nation’s financial system.

It doesn’t force the Citigroups and Goldman Sachses to break themselves into smaller institutions, or place the huge shadow banking sector under explicit oversight. Mr. Geithner tells of one early meeting plotting strategy on the law. Diana Farrell, a White House staff member, “made an impassioned plea for a more ambitious regulatory overhaul,” noting the irrational and convoluted system by which financial regulators are organized.

“But I pointed out to the room that in domestic policy as in foreign policy, there are wars of necessity and wars of choice,” Mr. Geithner writes. “Reform was a necessity. Reorganization felt like a choice that could mire the bill in the quicksand of interagency warfare.” The administration put broader overhaul of the regulatory system on the back burner.

In another sphere for which Mr. Geithner is frequently criticized, the administration’s perpetually underwhelming programs to help homeowners who were underwater on their mortgages, he writes this in his book: “We tried to do what we could within the constraints we faced. It wasn’t enough. But it was more than most people realized.”

That’s a fitting summary of Mr. Geithner’s tenure as the nation’s most senior economic policy maker. He is not your vision guy. He is not a person who might reimagine the very structure of Wall Street, or the world monetary system or American financial regulation as it ought to be.

Rather, he is a master of using the tools he has to their utmost, working within the system, avoiding unnecessary risks, and when a crisis strikes, keeping it from spiraling out of control.