I highly recommend both books by Sanjoy Mahajan (who has a background in theoretical physics), especially “Street-Fighting Mathematics: The Art of Educated Gues

I highly recommend both books by Sanjoy Mahajan (who has a background in theoretical physics), especially “Street-Fighting Mathematics: The Art of Educated Guessing and Opportunistic Problem Solving”. http://www.sanjoymahajan.com/works.htm http://www.sanjoymahajan.com/works.htm