The British National Archives http://www.nationalarchives.gov.uk/
holds papers relating to the French and Polish contribution to the solution of ENIGMA, particularly as revealed by General Gustave BERTRAND in his book, ENIGMA, and S A MAYER of the Polish Intelligence Service. They have an online exhibit.
The Alan Turing Digital Archive http://www.turingarchive.org/
contains digitized copies of Turing's
‘Treatise on the Enigma’. c. 1939-42.