I end up reading a lot of books and participating in research projects, much of which is not readily available on the net. The presentation by Dr. Cynthia Chappell deals with the biological basis for homosexuality and it's available as a one hour vdieo presentation http://www.pflaghouston.org/news/headline.htm
One of the better books on the market is Dr. Louanne Brizendine's THE FEMALE BRAIN in which she follows brain development from conception to menopause and juxtaposes the male and female brains as they develop.
The American Psychological Association has quite a bit of info on trans issues, the International Journal of Transgenderism has quite a lot of information, but I haven't always been happy with the quality of the things they have accepted for publication. They seen to have higher standards now than they used to have.
You can start here: http://emedicine.medscape.com/article/917990-overview on the emedicine site.
On gender.org you can start here: http://www.gender.org/aegis/index.html
http://www.genderpsychology.org/
http://www.gires.org.uk/Web_Page_Assets/Etiology_definition_signed.htm This site gives a run down on brain differences and has many sources for further study and research.
http://www.gires.org.uk/Text_Assets/maletofemale.pdf This is a synopsis of the research on the bed nucleus of the stria terminalus, with 41 references.
How much do you want? If you start reading and following references you can read thousands of papers from medical journals. Unfortunately most of this passes over the heads of the general population who are reluctant to expend energy learning something that they already have firm opinions about.