I've read everything I can find but I can't find out when the Allies found out about the parts of the Ribbentrop pact that gave the Soviet Union the eastern part of Poland and a chance to invade Finland. Was that known before 39 or was it realised after the war?
To the best of my knowledge, the Western Allies found out about the "spheres of influence" Secret Protocols of the Molotov-Ribbentrop pact after they captured the archives of the German Foreign Office in 1945. These were first published (in German, with select translations) in 1946. A preface to the book where selected translations appeared to general public (1948) notes that "use of the archives for intelligence purposes began immediately" after capture.
The book in question can be found scanned here: http://www.ibiblio.org/pha/nsr/nsr-preface.html
PS. the Western Allies probably didn't know much about the pact's contents before 1939 given that it was signed on 23rd September 1939 :).