I have learnt of the Conquest of the Incas by John Hemming, and Conquest: Montezuma, Cortes and the Fall of Old Mexico by Hugh Thomas but both seem like fairly long reads. Is there a single book by some historian that is somewhat shorter but containing both the histories of the Spanish conquests of Americas sufficient to whet my curiosity, even if reduced in details?
I'd highly recommend Matthew Restall's Seven Myths of the Spanish Conquest, which is reasonibly short and gives a good overview over early Spanish colonization. He looks at the Spanish campaigns against the Inca, Aztecs and other groups through the prism of later myths. I also think that the book takes into account native sources more than Hugh's does. It's in the AH book list where you could also find readings to build on this one if you like.