by Mary Katherine
Question: When writing a series should the whole series have a theme and then have each individual book have a sub-theme of its own?Answer
That would certainly be a sound approach, especially if by "sub-theme" you mean an aspect of the overall theme.
Of course, you can also give each book a separate theme that may nonetheless tie in with the theme for the series as a whole.
It's somewhat ironic, but the more complex the thematic side of a story is, the more realistic it becomes. In real life simple, general solutions seldom work with complex, individual situations - and readers know this instinctively. So having multiple thematic issues and considerations affecting characters' choices works really well in fiction.