Harrison Salisbury! Great - epic -definitive book (Just skip over the Russian poetry parts!)
I think they didn't swallow up Finland at the end of the war because there was no need to. If the USSR wanted something from Finland, resources, concession, etc they would get it. Finland signed a pact with USSR at the end of the war stating that while they were not allied with the USSR, if an attack on the USSR passed through Finland, then Finland would resist and repulse it. The term became known as Finlandization and is sometimes applied to Canada and our participation in North American Defence (I.E. NORAD, DEW Line etc.) In Finland and Canada's case geography dictated policy. One disgruntled Canadian diplomat disparagingly remarked that "we are all stuck on our flypaper".