"Sales Safari"[1] is probably one of the best ways to find which topics will sell.

General guideline from Russel Branson's "Expert Secrets": Pick one of the 3 major hot markets (health, wealth, relationships), pick a sub-market, then create a new niche in that sub-market (not recommended, but you might succeed competing with an existing niche).

"Stop your Divorce" [2] was written in the same way, and seems to have sold pretty well. (Also netted the counselor a lot of leads because his phone # was in the book.)

Interestingly, there's a counter argument to the advice presented in the "Parrot Secrets" book: [3]

[1]: https://shop.stackingthebricks.com/sales-safari-101

[2]: http://www.stopyourdivorce.com/

[3]: http://goodbirdinc.blogspot.com/2013/06/internet-fraud-targe...