What is your interest? Making models? Secrets to studying better. How to think better, Getting motivated, what to say when you talk to yourself, creative visualisation (athletes like to use that it's a way of practicing in between time)
World history, places go go, other cultures. hunting fishing,native wisdoms.
There really is no limit in the non fiction range. Some start looking at NLP and self motivation what makes us tick and what we can do to get better results from our efforts. Even learning languages and computing, design art chess.
Try PhotoReading a few books in the library and ask yourself if this topic is a go or no go. Then select a couple of go! and take one or 2 no go and just activate them and of checking how well you already know from a quick PhotoRead whether the book is for you. Purpose can just be something like I'm curious to know what this author says about...
Have fun, light a firework and see what colour it lights the sky 
Alex
One thing I really like about PR is that I don't get bored with the books like I did before I didn't PR. In fact, I think that boredom is what stopped motivating me to read as much. But when your mind is more captivated with photoreading, there is more incentive to read.