I don't remember who recommended "Writing and Selling Your Mystery Novel - how to knock'em DEAD with style", by Hallie Ephron, but it is one of the most in-depth books on creating characters I have ever read. The charts and graphs to be filled out seem such a small thing until you spend the time required. My heroine, Summer Meadows, is now my friend. I feel I've known her since we were children.
The drawback...I have to go back through my manuscript and "beef" her up a bit. This is all in chapter one. I can't wait to get to the rest of the book.