I wish I got the time to write al my book ideas but I don’t. Most of creative work is what I call “the art to see” and here is something for you to see. A hot bestseller right now is The Game by Neil Strauss a dating book for men in which the secrets in how to success getting dates with woman are developed into sort of a game. Al the woman I have been talking with about the book say that they “hate it” – but still they cant deny that it works (at least for getting dates, not relations). Here is the book idea for an bestseller to the opposite target group:

Write a book on how the manipulation in the dating “game” works. And how to see how is real and how is in it for the game and not the relation. Think a woman would be best to write this book, she could do interview with woman and man about how the game really works. A man could also write it with some journalist skills.
A book that has a demand is good, but it will not be a bestseller if it is not been read of the right target group etc. Seth Godin is sharing really good tips and tricks for you how to get your coming bestseller out on the market, read more.
Good luck with both the book and the dating research. Send me a copy when its out, I am source you will write it much better than I ever would done.