Although I don't have specific books in my mind, but such problem finding skills come from asking questions, always starting "what if ?", questions about making our life easier, people's life easier, company's work easier, just look and ask questions, also the best kind of ideas you come with, are the ideas that come from your own problems when you are the main user of that product, as you will keep always developing it.

Think in systems. Observe and measure everything you find interest in. Journal daily. Then find inconsistencies between the various elements and think how can you optimize them.

Be observant, think critically.

Yes. Keep challenging common knowledge. You can get pretty far with two basic thoughts:

1) It's always been done this way, what if it didn't have to?

2) This new thing I'm looking at is cool, could a similar change be applied to another space?

And of course, the ultimate definition (in my book) of the entrepreneur mindset is being a doer vs. a talker.

Presumably thousands of people had the idea that school, education, math could be explained better, maybe online. Only one person grabbed a camcorder and started recording hundreds of math lessons, creating Khan Academy.

Where to look for - Problems from your own life that you couldn't solve or suffered solving with existing solutions.

Books - Would suggest reading biographies.

A potential vector is work experience in a specific industry. With enough exposure you build a flair for "undiscovered" pain points.

The one big thing that was told to me: "Do something that scratches your own itch."