Hernan Cortes wrote and sent a series of letters to Charles V that may interest you. They are titled Cartas de Relacíon, but English translations are often titled Letters from Mexico such as this Yale edition.

Adventures In The Unknown Interior Of America, Álvar Núñez Cabeza de Vaca - This is a great firsthand account by the lone surviving officer of a Spanish expedition to conquer Florida.

My favorite super easy, super engaging overview of conquest is Restall's Seven Myths of the Spanish Conquest. This book is a great place to start because it breaks down many of the popular history tropes and sets you up to place further reading in historical and historiography context.

Shirley and Richard Flint have written very extensively about the Coronado expedition into the US Southwest, which was the first Spanish expedition other than the exploratory ventures by Marcos de Niza that led to the Coronado expedition.

Really anything they have put together is very good, but No Settlement, No Conquest: A History of the Coronado Entrada by Richard Flint is probably the best overall historical narrative of the entrada.

I'm going to bed now but tomorrow I'll post some. What do you exactly want to read about, the discovery, the conquest or the colonization?