Great stories require believable characters and they must interact properly with other characters.
Since relationships are the focus of my fictional work, this is a really important aspect of writing to me.
And ultimately, relationships are the number one thing in life as well!
So some fantastic books on the subject of relationships-
The Five Love Languages and The Five Love Languages of Apology by Gary Chapman
Personality Plus by Florence Littuaer
Men Are From Mars Women Are From Venus by John Gray Ph.D.
How to Start a Conversation and Make Friends by Don Gabor
The Friendship Factor and Bringing Out the Best in People by Alan Loy McGinnis