This one contains rather more letters than the later volumes. Provides some interesting insights.
Into Orwell for one. His unthinking biases, including his assumption that Socialism is the only right way -- oddly enough while commenting on how government inspectors who refused to let farmers hire hop pickers without providing "proper accommodation" only caused suffering among the itinerant workers, and observing that a heavily Conservative city is effectively Socialist with the amount of crony capitalism going on. (There are more about this assumption in the later volumes.)
Though, re-reading it after reading the other three volumes puts a certain amount of glee in the passages where he's complaining that Fascism and capitalism are really the same thing, because then you have read his arguments against that position. He sticks to it in this book however until the last essay in, where he admits to having argued against the war, but having dreamed just before the Russo-German Pact, he realized that he would support the country if war came. (He understates his earlier opposition; that was exactly what he predicted many people would do, and he derided them for it.)
A number of reviews give an interesting glance at literature of the time, and earlier. There's a fair number about the Spanish Civil War, in which case his personal experience is brought to bear on the subject matter of the book. (He has nothing from while he actually served, but wrote about it after.) He wrote an essay on Dickens that has interesting insights -- not all into Dickens, some into Orwell.
Also, he has some essays on life in England, generally among the very poor, with some interesting anecdotes. Like the woman who always kept herself respectable -- that is, always wore a hat, not a shawl -- and how one time she tried to get some charity and was told they were for people really in need, and if she can dress like that, she must not be. She rather resented it.