Duncan Hallas book The Comintern starts off promising with Hallas explaining that his perspective, that of a 'revolutionary socialist', is underepresented when it comes to books on the Comintern or the Communist International. Hallas doesn't say outright, but what he means by revolutionary socialism is that he is a Trotskyist in orientation. In terms of the text this means that he sees Trotsky as the inheritor of Lenin's revolutionary legacy and Stalin the degenerate.The book follows this course pretty consistently as it traces the history of the comintern and the parallel history of the USSR, as well as the german revolution, and other movements occurring at the time.
If you are only looking for a barebones trotskyist history of what happened when and who was involved and what went wrong (Trotsky says it was Stalin!) then this is a decent overview. However, if you are looking for more than a very surface level analysis when it comes to explaining various movements and the logic of policies then I would look else where. Hallas' analysis is very weak and he glosses over many of the policies while asserting definitively which ones were correct and which ones were wrong. For instance, he explains early that the 'United Front' polices was correct while the later 'popular front' was a disaster. He doesn't clearly explain the difference between the two policies that appear to be the same and it makes it seem like he doesn't approve of the policy simply because it was Stalin putting the policy in place instead of Trotsky.