It has been a while since I read 39 Steps but you are reminded how capable Buchan is of weaving a plot with pace and mystery.
Having spent 80 odd pages laying the foundations the story goes up a gear and Lammas starts to come into his own as the true evil of the scheme to undermine the British government becomes clear. Lammas is the one who has the strange encounter with one of the main victims of the plot and he picks up the burden of chasing down the arch plotter Crammer and releasing Mrs Crammer from certain death.
Up to this point the story has been muddied with love interests of two sons of significant men but for now that is cast aside as the main plot takes over. Buchan splits up the characters and has the action coming to a conclusion from several different directions. He also uses coincidence with great effect as well as making his characters walk the tightrope between being believed and being seen as mad.
As things start to move towards some sort of conclusion the reader is gripped expecting the plot to go in a certain direction but gladly letting Buchan take you on that journey.