Adam Bruno Ulam was a Polish-American historian and political scientist at Harvard University. Ulam was one of the world's foremost authorities on Russia and the Soviet Union, and the author of twenty books and many articles.
I imagine this is probably quite a rare book these days - this 1951 publication offers a very insightful contemporary account of not only the Soviet-Yugoslav split, but the general conditions of Eastern European communism during and immediately after WWII.
The book itself is the product of an outsider looking in, being an American research effort undertaken at the start of the Cold War, and has obvious limitations in this respect. However, the author has plainly done his homework to the best of his ability, contrasting a thorough reading of the communist press with what first hand accounts of the war and its aftermath he had at his disposal at the time.