Dietary Reference Intakes for Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline, National Academy Press, 1998.
Hulda Regehr Clark, PhD, ND. The Cure for All Diseases. Chula Vista, California: New Century Press, 1995.
Homocysteine in health and disease, ed. Ralph Carmel and Donald W. Jacobsen, Cambridge University Press, 2001.
H. Winter Griffith, MD. Vitamins, Herbs, Minerals, & Supplements. USA: Fisher Books, 1998.
The American Dietetic Association and the National Institute of Health have also published studies regarding digestive issues (across all types of eaters) that can hinder absorption, as well as how increased age affects absorption. I'm sure there are links on their respective sites.