The AskHistorians Books and Resources list has a small section on Spain and the Spanish Empire that you could check out.

Although a bit dated, Lockhart and Schwartz' Early Latin America is a great resource for Spain and Portugal's empires in the Americas.

There is also Maclachlan's Spain's Empire in the New World.

For more Spain centric works check out Ruiz's Spanish Society, 1400-1600 and his Spain's Centuries of Crisis: 1300-1474 provides good background to the run up of Castilian expansion after 1492.

Kamen has a number of works Spain, 1469-1714: a society of conflict and Empire: how Spain became a world power, 1492-1763

And Hugh Thomas has written a number of works Rivers of Gold: The Rise of the Spanish Empire, from Columbus to Magellan, The Golden Empire: Spain, Charles V, and the Creation of America, and Conquest: Cortes, Montezuma, and the Fall of Old Mexico