Secret Power available online
Secret Power, investigative reporter Nicky Hager's 1996 book on New Zealand's SIGINT agency, the Government Communications Security Bureau (GCSB), has just become available online as a free download.
The GCSB is one of the five partner agencies in the UKUSA intelligence community. Secret Power was and is by far the most complete look inside the structure and workings of a UKUSA SIGINT agency, and it has also been the best single source of information on the infamous "ECHELON" system.
Matthew Aid's soon-to-be-released history of the NSA, The Secret Sentry, will provide a far more complete look at the history and workings of NSA and the UKUSA community as a whole, but I doubt that any agency will ever again go under the microscope quite the way that GCSB did in Secret Power.
(HT to Cryptome.)
Full disclosure: I am one of the many people listed in the author's acknowledgements.