Actually it was a bit difficult to find something immediately. But I did stumble across this:
Science of Desire
I haven't read it at all, but it looks like it's an account from Dean Hamer about his research into the gay gene.
The Biology of Homosexuality
This one is also looking at "...embryonic endocrine and genetic phenomena..." in its explanation of homo/heterosexuality.
There were a few books about conversion therapy etc.. which might also cover some theories about homosexuality from a medical or psychological point of view, but I'm guessing their focus is more on conversion and possibly less interesting to you.
This was just a brief search I did in Amazon. The subject isn't terribly interesting to me so I don't have "in the know" recommendations to make.