The Linus Pauling Institute's Micronutrient Information Center (MIC) is a source for scientifically accurate information on the functions and health effects of all micronutrients (vitamins and nutritionally essential minerals).
The National Institutes of Health provides fact sheets about a variety of dietary supplements.
Books and eBooks
Search Tripod for more books about vitamins in general or the specific vitamin you are researching. For more targeted results, do an advanced search and search your keyword as a subject.
Nutrition at a Glance (2013)
by
Sangita Sharma, ed.