The treasures of the CIA reading room
Some of the most striking cultural differences between US and UK intelligence are evident from their respective websites. While British agencies are not subject to freedom of information, the CIA has an extensive online archive in its FOIA reading room.
Although it includes internal records of a kind its difficult to imagine MI6 ever releasing, it is also a useful repository of material originally published elsewhere. There are many intelligence-related news articles that might otherwise be locked behind expensive paywalls, as well as articles from CIA-critical magazines such as Counterspy and Covert Action Information Bulletin. This 1980 edition of the latter includes an article by Phil Kelly on former MI6 chief Maurice Oldfield's ill-fated mission to Northern Ireland, as well as the controversial 'naming names' section.
While CAIB has its own web archive, Counterspy seems a bit harder to get hold of. It is ironic, to say the least, that the CIA is hosting some of material which provoked the 1982 Intelligence Identities Protection Act, even if it is water under the bridge by now.
There is even the odd book, such as this study of post-war MI6 which turned up when I was researching my previous post.