There is a book called Hypericum and Depression. It is available in full text with updates at www.hypericum.com . there are also sources (supposedly reputable) listed. Regarding St. John's Wort  and breastfeeding the book says safe while breastfeeding, but also states "the resulting increase of dopamine increases the production of a prolactin-inhibitory factor, which in turn diminishes the secretion of prolactin...as well as an inhibiting effect on the menstrual cycle and libido."