I've always enjoyed the Zorro legend, but had never really thought too much about his background. Allende fills in that hole admirably. Eminently readable and an exciting ride thru turn of the century (the 19th century) California and Spain, this book just sings.
Allende occasionally steps out of the role of dispassionate narrator and brings herself into the story - "And now, dear reader, let me tell you what happens next". The first couple of times this happens are a little jarring because it's totally unexpected, but by the end of the book, the reader feels that s/he is sharing an inside joke with the narrator.
The notes at the end of the book explain that Allende was approached by Zorro Productions (owner of the character's copyright) to write this book. She almost turned them down, but I'm really glad she didn't. This is a beautiful book.