This book is an excellent tool for authors. I have used it to help define my characters in my last four manuscripts, and it has helped me "get to know" them before I write the first word.
Are You My Type, Am I Yours is a psychology book that contains nine basic personality types. It details the best and worst attribute of each type, gives a personality inventory and talks about social and relational characteristics. One of the things I like best is at the end of each personality description, it talks about what each type likes and dislikes in the others.
This is fantastic fodder for the fiction writer. For example, in my manuscript, Captured by the Pirate Laird, I identified my heroine as a perfectionist and my hero as an achiever. Ones and threes are not connected in the enneagram, which can make the relationship a bit more challenging.
To give a tidbit of an example of what I'm talking about, perfectionists are ethical, idealistic, productive, etc. They like achievers because they work hard and accomplish a lot. They have trouble with achievers because they are overly sensitive and withdraw when confronted.
See how this helps define your character's personality, and even better, conflicts?
I. LOVE. THIS. BOOK!