Loved ones of those who suffer from Borderline Personality Disorder often have questions about the “best” way to help. The best help can be getting the person you care about in therapy with a qualified therapist. You may also want to read about the disorder to aid you in understanding what he or she is dealing with.

Stop Walking on Eggshells. There’s no more apt title for the experience of living with someone who has Borderline Personality Disorder. Some of the concepts are simplistic but a good starting point.
Stop Walking on Eggshells Workbook. The companion guide to the above book.
I Hate You. Don’t Leave Me.. This book is getting old, but it has been helpful to many people. It was written prior to the current thinking about BPD became popular but you may still find it helpful.