This was an audiobook that was written evidently for a much younger audience than I represent. The book was read by Kilmeade, and he did it in such a strident and breathless voice that I couldn't stand to listen to it. Worse than this though, the facts were presented in such a biased and fanciful fashion that I found myself having a hard time swallowing everything he said. It felt much more like listening to florid fiction than to historical fact.
The secret six were actually known as the Culpeper ring, named after a Virginia County. They were spies who fed information out of New York City to Washington about the activities, movements, and plans of British troops in NYC. The main two members were Abraham Woodhull and Robert Townsend. The best information they got was when they laid hands on a British naval Code handbook. That was less through spying than from luck, but it served the French Navy well.
While these guys (including women) did provide other valuable information, the value of some of their activities was debatable. It's arguable that the defeat of the Brits and surrender at Yorktown did more than any spies did, and this victory was brought about as much by the French and Spanish as it was by the US, if not more so. Cornwallis could well have withdrawn rather than surrendered had the port not been very effectively blockaded by the French.
The secret six were spies for the revolution forces, which side was consistently presented as upstanding, brilliant, heroic, and fine, whereas, of course, the British were evil villains. This was exploited most obviously in the report of the British prison ship HMS jersey, which was pretty brutal, but this was war and it was in the early 1780s, when people were hardly the most civilized and no Geneva convention existed. Additionally, the revolutionaries were considered traitors, so the Brits were not very much disposed to treating them kindly. Not that Washington had many prisoners to exchange anyway, since the British captured far more US forces than the other way around. That doesn't make what happened palatable, but it does provide some context that this helter-skelter account fails to do.
Another thing this story doesn't make clear was that Washington, who could have exchanged prisoners, was disinclined to do so because he didn't want to exchange professional British soldiers for civilian volunteers and conscripts! He didn't consider it a fair exchange. How brutal was that? Remember these were the guys who were fighting for the rich folk who didn't want to pay taxes. That's what today, we call Republicans.
The rich were the guys who claimed they wanted the vote, but none of the guys fighting on the front line ever had the vote! Only about 6% of the population were eligible to vote in 1789! In short, the pretext of the revolution was bullshit, yet those who were wealthy were not the ones dying en masse on the front lines or being interned (and interred) in the HMS Jersey! How long did it take for American Indians to get the vote? For African Americans? For women? This wasn't a fight for freedom - it was a fight for the rich, and the poor paid the price on both sides. When that emancipation was truly sought, it started a civil (read not-at-all-civil) war a century later.
So my take on this is that if you're looking for an historical account, don't look here. If you're looking for an hysterical account, then this is the audiobook for you!