Just finished 'George Washington's Secret Six - the Spy Ring that Saved the American Revolution" by Brian Kilmeade and Don Yeager.
Very enjoyable, an important part of our history. All this was new to me. Like many popular historical books, this one has been criticized for taking legend or someone else's theory and presenting it as fact, without adequate source notes. I don't know enough to judge that, but it does not seem to significantly change the story of what these people really did. The reviews for this book are mostly favorable.
One of the things I liked was the bibliography. While the list was a bit shorter than most books, he did add a sentence or two describing the books and papers used. I found this helpful since I would like to read more about these people.
If you are thrifty like me, you can probably pick up a nice copy at a used book store or on line retailer for a good price.