read university textbooks, i.e. undergraduate books outside your field. Also see supplementary material such as Cambridge's "A Student's Guide To ..." : https:

read university textbooks, i.e. undergraduate books outside your field. Also see supplementary material such as Cambridge's "A Student's Guide To ..." : https://www.cambridge.org/core/series/students-guides/DE92BAAE79DC05E3E353080483FF05A0 https://www.cambridge.org/core/series/students-guides/DE92BA...