How does a scientist decide which problem is worth working on? That was the question which faced Mlodinow when he became a researcher at Caltech. How could he live up to the expectations placed upon him? Fortunately he had Richard Feynman down the corridor to talk to, and this book is the result of their many discussions. The answer seems to be that if you need to ask the question then you shouldn't be doing physics - indeed Mlodinow eventually gave up his career in the subject. Rather you should be driven by your desire to sort out inconsistencies in the current state of things. Feynman's Rainbow is a highly readable book - I read most of it in one go, and I can recommend it to anyone who wants to find out how scientists decide to do what they do.
Feynman actually had the same sort of problems when he started research - awe of the 'Monster minds' which surrounded him and fear that he couldn't get down to working on anything worthwhile. But you'll need to read Surely you're joking Mr. Feynman to find out about that. In Feynman's Rainbow Feynman clearly dislikes the idea of nursmaiding a new researcher along, but he was willing to go on talking with Mlodinow and revealing many details of what motivated him in the process.
One thing that I found interesting was the counterintuitive way in which your sense of urgency changes with age. For a young researcher there is a need to do something significant within a couple of years. On the other hand for Feynman, although he knows he is dying of cancer, there is no sense that he must rush to do what needs to be done.