Do you have a particular period that you are interested in, and are you more interested in a very broad overview, or would you be interested in more specific recommendations? And how long of a book can you digest? My single favorite book on the history of Spain is Elliott's Imperial Spain which covers the period from Isabella and Ferdinand, to just beyond the Thirty Years' War. It covers the rise of the Spanish Empire, her struggles both internal and external, and covers the era of Don Quixote.

Have a look at the Amazon preview and tell us if this is too big of a book for you. If you are going to see the Prado and other museums this book will help you enjoy your visit.