Yet another surprise revelation suggests strongly that the FBI’s probe of Hillary Clinton’s e-mail mess was anything but a by-the-book investigation.

House Oversight Committee Chairman Jason Chaffetz (R-Utah) said he learned only Friday that the Justice Department gave immunity deals to Clinton’s former chief of staff, Cheryl Mills, and two other aides. That brings to five the number of Clintonistas who got a pass in exchange for testimony and/or information.

But what makes it especially significant in Mills’ case is that she was allowed to sit in on Clinton’s FBI interview, asserting attorney-client privilege on Hillary’s behalf. This, even though Mills was herself a witness, even a potential subject of, the investigation.

Indeed, she was a key player in the process that deleted tens of thousands of Clinton e-mails — many later found to be classified — before the FBI could subpoena them.

Mills’ lawyers say her immunity deal was limited to the contents of her laptop, and was given because of ongoing debate over after-the-fact classification.

Prosecutors normally strike such deals only when they can’t get the information any other way. But if Mills refused to turn over the computer without any immunity, why couldn’t the FBI just subpoena the laptop?

After all, as Chaffetz notes, “immunity deals should not be a requirement for cooperation with the FBI.” Yet in the Clinton case, he noted, “the FBI was handing out immunity deals like candy. No wonder they couldn’t prosecute anyone.”

Frankly, it’s amazing that Hillary didn’t get a deal herself. Then again, it’s become clear that she didn’t need one.

President Obama publicly declared months ago, at the start of the FBI investigation, that Clinton had done nothing wrong. As we said at the time: The fix was in.