This webpage will give all you need to know about the science related to TS - what it is and is not, why it's there, etc.
That teaches about the differences about male/female brains. It might provide a little info about transsexuals having the brains of the opposite sex.
Also visit the APA website about sexuality. It talks a little about transgender and what the term means, talks about the different groups of TG, etc. I do not know the address... I used to have the address on another forum site, but for some reason it's gone.
I hope that helps.