Lots of professional research studies done internationally. Keep in mind that these studies may contain graphic images, particularly if they talk about surgery. They also have a document that details their recommended standards of care. Many websites they link to can be used for reference as well.
This website is particularly useful for explaining transgender people to parents as all their research has been done professionally, and done within with universities.
Description off their website:
The World Professional Association for Transgender Health (WPATH), formerly known as the (Harry Benjamin International Gender Dysphoria Association, HBIGDA), is a professional organization devoted to the understanding and treatment of gender identity disorders. WPATH Mission Statement. As an international multidisciplinary professional Association the mission of WPATH is to promote evidence based care, education, research, advocacy, public policy and respect in transgender health.