There are several books I use for developing my characters.
One is Personality Plus by Florence Littauer. It covers the four basic personality types - Choleric, Phlegmatic, Sanguine, and Melancholy. It is easier to determine a character's reaction to life when you know his or her personality type.
I also use The New Birth Order Book by Dr. Kevin Leman for the same reason. The four birth orders line up with the four personalities, enabling me to properly place a character with the correct amount of siblings.
I also rely on Gary Chapman's book, The Five Love Languages. There are five means by which to express affection to family, friends and loved ones. If a character's love language is acts of service, I know they will not be a touchy-feely kind of person!
All three of these help me develop my characters!