They don't go much into how they invaded Poland two weeks after Nazi Germany did or their near disastrous war with Finland in the winter of 1939/40.
I think “chummy” is probably the wrong term to use. From what I have read, both men despised each other. However, both were willing to making deals with the devil to achieve their political aims.
The reason Stalin made the deal with Hitler, was because Hitler let him have half of Poland.
The Brits and French, mainly due to Polish opposition, would not let Stalin into Poland, and therefore the negotiations broke down.
In his book, The Mortal Danger, Aleksandr Solzhenitsyn slammed westerners like you for equating "Russian" with "Soviet." The Communists were occupiers, oppressors of the Russian people and not to be confused with one another.
Imagine how you would recoil if foreigners associated you with the democrat party.