Nursing Mother’s Companion, by Kathleen Huggins (includes a great section in the back regarding the safety of drugs during breastfeeding)
Bestfeeding, by Suzanne Arms
Womanly Art of Breastfeeding, by La Leche League
Essential Guide to Prescription Drugs, by James J. Rybacki & James W. Long (has list of 2000 prescription drugs, listing any interactions with herbs and whether safe or not for pregnancy and lactation)
Medications in Mother’s Milk, by Thomas Hale (can be ordered on-line)
Wise Woman Herbal for the Childbearing Year, by Susan Weed