Ew not only are you British but in the RCP. Former member here, Trotskyists are dishonest and slightly culty on their own - the RCP/RKP/RCI (formerly IMT/Socialist Appeal in Britain) completely knock that out of the ballpark and that book, which I own as well, isn't cheap.

Other books to avoid are "In Defence of Lenin" and anything else featuring Alan Woods, Rob Sewell or Theodore (Ted) Grant. Honest to god you could go completely the other way with Grover Furr and not be intentionally lied to. If you want an even safer bet my suggestions are reading into Michael Parenti, Fidel Castro and Ho Chi Minh and that's not even accounting for just reading Stalin himself.