"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]
When I was about 16 I collated a load of information on looking after your bearded dragon and sold the eBook on eBay - maybe made around £150 overall. I was chuffed at the time.
Referencing this sort of thing: http://www.parrotsecrets.com/
An explainer: https://www.cringely.com/2009/03/14/parrot-secrets/