Title for my fictional non-fictional work

Actually there are many ideas I have, which I want to write about and publish.
Before I can do that, though, I want to sort out my ideas and titles of my work.
There are various ideas I have, but I would like to know what readers want to read, especially in the non-fictional field.

Is religion and spirituality a good topic to write about? Or are there already too many books on it?
How about simple maths and philosophy? This would be another alternative idea. I want to write about a new concept, concerning philosophy in maths classes, without disconnecting maths language from everyday language.

What is your idea and your input about it.
Thanks