Just to correct a reader's review, this book is not a series or collection of letters to heads of state all over the world. And it was not written to defend the US Constitution to other world leaders, for this volume was released before the US Constitution was written, and months before the Convention that wrote the US Constitution even met. Furthermore, the use of "letters" instead of chapters is a stylistic device.
Adams' three volume work was written to defend the constitutions of government that had been adopted by the several states after their separation from Great Britain. Several of these constitutions, which utilized mixed and balanced features, were being criticized in Europe because they too much resembled the British government.
Governments consisting of a single assembly were all the rage at the time, for they were viewed as being very "democratic," and a refreshing alternative to the monarchical governments that so pervaded Europe.
For example, Benjamin Franklin's Pennsylvania constitution, creating a single-assembly government, was heavily praised and considered modern and innovative, while John Adams' Massachusetts constitution, creating a mixed and balanced government, was highly criticized and considered outmoded and merely a copy of the government of Great Britain.
In typical Adams fashion, his work uses examples from history to demonstrate that single-assembly governments were not new or innovative, but had been tried and were all miserable failures, while properly mixed and balanced governments were the most stable, most free, and most long-lasting governments that ever existed.
Indeed, within a few years of the publication of Adams' work, the entire world would have an example of the turmoil and blood that result from a single-assembly government by watching the results of the revolution in France.
It should also be noted that Pennsylvania abandoned its single-assembly government by 1790, adopting instead the mixed and balanced government it has to this day. And John Adams' much maligned (by Europeans) Massachusetts constitution, with its mixed and balanced features, remains the oldest written constitution still in use IN THE WORLD. Indeed, it served as a model for the US Constitution, and Adams' work was much read by the delegates at Philadelphia who created it.
I am, however, happy that the reviewer found Adams' work to be an "enjoyable read," for it is indeed a classic, and timely in any age.