As someone who was preparing for International physics Olympiad in heydays in senior year of high school, reading Feynman Lectures of Physics (especially part-2 on Electrodynamics), made the real difference. I was reasonably prepared competing at regional level, but Feynman's way of prodding the deep questions made me realize how many cracks I had missed. It also helped me build the skill of stopping at a reasonable point - its incredibly easy to keep grappling at really hard problems which may never lead to solution with existing skillsets. Judging what you don't know & incapable to solve is very important and perhaps applicable to all disciplines.