Most of them would be in Spanish or German, as tou may imagine. I will give you a few books off the top of my head:
Fray Antonio de Guevara, Epístolas familiares. Guevara was a very wise man, who was in contact with many people from the Emperor's court.
Francisco López de Gómara, Las guerras de mar de Carlos V. Gómara, chronicler on Hernán Cortés' payroll, wrote a very interesting book detailing Charles V's naval campaigns.
Gonzalo Fernández de Oviedo, Relación de lo sucedido durante la prisión del rey Francisco de Francia. Oviedo was an official chronicler of the Indies, but he wrote many books, this one telling what happened in the Charles' court between the capture of Francis I of France and the war in Hungary in 1533.
Alonso de Santa Cruz, Crónica del emperador Carlos V. A general chronicle of the reign of Charles V, written during his reign and a bit later.
You can even have some texts written by Charles V, like the Instrucciones de Palamós, a short corpus of letters by Charles to his son Philip in 1543 as he was appointing him Regent. In these letters he instructs Philip on how to act as regent and how to be King.
If you want official documents, you can access lots of them through PArEs, filtering by archives. Some of the documents you may find are diplomatic correspondence and instructions (mainly in the Archivo General de Simancas, section Consejo de Estado).