Disclosure: I read a lot of historical fiction. Love the stuff. From Regency romances to the based-in-real-life, it's one of my favorite genres. In particular I can recommend Judith Tarr's books set in ancient Egypt and Alison Weir's books set in Tudor England.
I can also recommend The Last Queen. This was my first foray into Spanish history; it's the fictionalized life story of Juana the Mad, one of the daughters of Isabel and Ferdinand. (Her younger sister, mind you, became Catherine of Aragon--Henry VIII's first wife.) Gortner tells the story in compelling form; the book was extremely difficult to put down. And at the same time, at those moments when Juana was truly trapped by the politics around her, I almost wanted to throw the book across the room. Her frustration was that real, her predicament that moving.
A little background for those who know even less than I do about Juana's life (I found this book highly educational): she was married off to Philip of Habsburg, who was archduke of Flanders. By rotten luck, she ended up as heiress of Spain when her brother, older sister and older sister's son all died. Most people would count that as great luck, but it worked against Juana. Philip was moved to extreme ambition and attempted to take Juana's throne, and their heretofore happy marriage became violent. Juana suffered severe anxiety, and with reason--her husband effectively imprisoned her in an attempt to control her and use her in his own quest for power. It didn't help that Juana's family had a history of "madness" (most scholars now suspect it was manic depression).
Makes for a great story, and Gortner, who is himself half Spanish and was raised in Malaga, takes the threads and weaves them to advantage via his fine prose. One hears Juana's singular voice echoing through the ages. Sure, it's only an author's best guess, since all anyone can go on are historical facts and primary source accounts. But Gortner's "best guess" is a fine one indeed, at least when it comes to entertainment value.