Richard Aldrich has written a historical tome 'GCHQ: The Uncensored Story of Britain's Most Secret Intelligence Agency', which has been well reviewed:http://www.amazon.co.uk/GCHQ-Richard...mm_pap_title_0 and for the USA:http://www.amazon.com/Gchq-Uncensore...ligence+Agency

There is also a sub-website on his book plus other sources:http://www2.warwick.ac.uk/fac/soc/pa.../lectures/gchq