It's not easy. The primary source of income for professional historians is the salary provided by the university where they teach. If you are lucky enough to get a tenure track job, you are basically set and secure, but adjuncts are paid very little and there is no job security. As far as publishing royalties go, historians make very little money from publishing academic books. A historian is likely to make significantly more money from annual book prizes given out by different institutions (if the person is good/lucky enough to win one). There are other ways to make money on the side including independent research or working with a research institution/archive, but once you are a fulltime professor, teaching and publishing generally become your only focus.