I have written this for people who want to know what counterintelligence is, not what it ought to be, and for people who may be interested in it as a trade or profession.
The book is about what professional intelligence officers call "tradecraft", specifically the craft used in the trade of counterintelligence. It is not about politics or policy or communism or anticommunism or justice in the Third World or human rights or religion, although these affect the trade of counterintelligence just as they do the trade of stock-brokering or oil exploration or journalism. They will be mentioned occasionally, and my concerns about them will be evident, but only as they are elements of the enviroment in which counterintelligence functions.
My thirty-odd years working in counterintelligence have all been spent as an American official, but I have worked much of that time with the counterintelligence officers of other countries. I believe this book will be useful to readers not only in the United States but also in other countries allied with the U.S. and in some non-allied, non-hostile, where espionage and terrorism occur.
To illustrate various points I have cited many actual cases. Some of these have been written about publicly elsewhere, with varying degrees of accuracy, and some have not. Those which have not yet come to the attention of journalists, historians or writers of fictional documentaries I have altered (in counterintelligence jargon, "sanitized") by changing names, dates and places. I have done this to protect myself and to protect what American law calls "sources and methods" from hostile action. I have made some changes and deleted some material at the request of the American Central Intelligence Agency, which has reviewed the contents patiently, promptly and thoughtfullly. What I know about the spy business I learned as an official under oath to my government, and therefore what facts I know about the spy business are the government's property, not mine. The opinions are my own, and CIA neither endorses nor condemns them.
--- from book's Introduction