(Photo by Ann Forsyth)
Focusing on gardens and other landscapes, with a particular emphasis on gardens in healthcare settings, the web site of the Therapeutic Landscapes Network is a well-designed and maintained resource. I have found the following sections to be particularly helpful:
The evidence-based design section at http://www.healinglandscapes.org/ resources-ebd.html includes online searchable databases, books, and articles.
For further reading, the Design for Health web site deals with planting and mental health at: http://www.designforhealth.net/resources/mentalhealthissue.html