"King Ottokar's Sceptre", first completed in 1939, is a pretty entertaining Tintin adventure, featuring a plot to overthrow a king in the psuedoeuropean country of Syldavia, and the first Tintin story to feature opera singer Bianca Castafiore. There's a few allusions to the threat of the real life Nazi regime, which is interesting giving the time it was first written.
I thought there was pretty interesting theme running through this story, of doubles. A man and his double are a big part of this plot, while there a lot of things that happen twice (Tintin's window being broken twice, the Thompsons having their hats ruined twice, the Bordurians planning their second invasion in history, etc). Once again, being written on the verge of the world going to war a second time, this is an interesting theme to have. For a fictional country, Herge has really developed Syldavia, and the travel brochure Tintin reads (which the reader can read along too) was an interesting fictional overview of the place. (Extracted from Wobu Zhizao II)