A good introductory book I'd say would be Michael Wood's Conquistadors. It details accounts of four different conquistadors: Hernan Cortes' conquest of the Aztecs, Francesco Pizarro's conquest of the Incas, Francisco Orellana's hunt for El Dorado, and lastly Cabeza de Vaca and his American travels. The book was created in accompaniment with a TV series of the same title presented by the author for the BBC, which I believe can be found online.