The poacher, the sheep farmer, etc are guests on Walter's radio programme. "Open Air" means outdoor life here. These guests are all connected with outdoor activities. "Stock" here means quantity. "a commodity in which Walter had lately taken so much stock as to make it a virtual Whitmore monopoly" - Walter has done so much outdoor life that it has almost become his signature. Please note that Walter's surname is Whitmore. "…as to make it a virtual Whitmore monopoly" is a figure of speech: he is virtually the only broadcaster who has been doing so much outdoor life. Half of the programme features his conversation with the guest. During the other half he talks alone. This is the format he follows. Seven, by now you should realise that the book is too difficult for you. It is a well-wriiten book, but it is steeped in English life of a particular period (circa 1930s - 40s). Moreover, it is written in such a way that details are interwoven throughout the book. To understand it completely you need to be able to spot the interweaving threads. You are not used to this kind of writing. British life is also very alien to you. That is why it has been such a struggle for you.

“Open Air " is likely the name of a radio programme, perhaps on the BBC; it is neither likely for it to be a kind of commodity nor for anyone to to trade stock in it; even less possible for any radio programme to be anyone's stock trading monopoly, virtual or not. Apart from that, what do you make of this: "Walter compèred the Open Air in the shape of a poacher, etc.". Did he change his shape? A shape-shifter on radio? And this "For the latter half of his time Walter merely talked." What else can you do on a radio programme but talk? Was he singing at some point in time? What is "merely talked"? When I see a puzzling passage like this one, I know it's time to throw the book away and start reading one by a better author. Please don't miss out on the old classics: H. Rider Haggard, Edgar Rice Burroughs, G.K. Chesterton, Conan Doyle, Bertrand Russell (the best English phrasing I have ever read), Somerset Maugham and so many others.

Hi Seven! Stock means the amount of shares of the value of a company. A monopoly means a company that basically (or virtually) has no competition.