'Why Love Matters' (Sue Gerhardt) is one of the best books as it has incorporated research findings in a very readable way. Also...

The Continuum Concept: In search of Happiness Lost' (Jean Liedloff)

The Natural Child: Parenting from the Heart' (Jan Hunt)

'Attached at the Heart' (Barbara Nicholson and Lysa Parker)

'The Attachment Parenting Book' (William Sears and Martha Sears)

'The Unborn Child' (Roy Ridgway) - for those who like evidence-based scientific research.