The Central Intelligence Agency has released a newly declassified version of its closely-held internal history of the Berlin Tunnel Operation (pdf), which was an effort in the mid-1950s to tap into Soviet communications through a tunnel constructed in the Soviet sector of Berlin. The operation was famously compromised by a Soviet mole in British intelligence before it even began.
The official CIA history of the operation was prepared in 1968 and published — in two copies. A declassified version was finally approved for release in February 2007.
See “Clandestine Services History: The Berlin Tunnel Operation, 1952-1956,” 24 June 1968.
CIA internal histories are a largely untapped resource since the Agency has been slow to declassify and release them.
A previously published CIA account of the Berlin Tunnel operation, which includes links to limited excerpts from other internal histories of the episode, is here.
A clandestine services history of the 1953 coup in Iran was leaked to the New York Times in 2000, after the CIA refused to declassify it. The document is available from the National Security Archive.