CDC'S HIV/AIDS Surveillance Reports - You can search the site for "transgender" - has a ton of studies and articles on HIV and trans groups, in Adobe Acrobat files.
SIECUS - The Sexuality Information and Education Council of the United States is a 35 year old nonprofit organization, dedicated to affirming that sexuality is a natural and healthy part of life. SIECUS develops, collects, and disseminates information, promotes comprehensive education, and advocates the right of individuals to make responsible sexual choices.
Transgeneder Positive - From the Nov. 1996 issue of Body Positive, an article that discusses definitions, empowerment, medical services, hormone therapy, sexual realighment surgery.