The second part of a 2-part series, this comes after 'the seven crystal balls' and was first published in 1949. I think this is on a par with the excellent 'Tintin in Tibet' in being a good all-round adventure story, pretty scary in places (at least for a 7-to-10-year old), respectful of other cultures and anti-racist. The drawing and coloration is top-notch, the setting (Peru) is great, the quest is noble - it is all here, it is all good. My only quibble is that the trick by which our heroes escape from a horrible fate (I can't give away the plot, of course - don't want to spoil it for anyone!) is rather unbelievable - the Inca were pretty sophisticated where that particular subject was concerned.
Altogether an excellent story. If you are new to the Tintin series, I would start with Tintin in Tibet, the Cigars of the Pharaoh, or Land of Black Gold; but as a follow-up, this 2-parter is excellent fodder for kids and the young-at-heart alike.