I gave this such a high rating partly because I felt that it gave some strong insight into trade union and political history.
For a start, the book was the first to show me that the working class was invented by the trade unions in order to create a cohesive movement. Before the trade union movement, there were multiple working classes and they didn't see eye to eye. The part about Marxists taking over trade unions in the Fifties felt close to home, like the battles between the centre and Left of the Labour party. An underrated book.