Almost everyone learning graph algorithms or combinatorics soon comes across Seven Bridges of Knigsberg problem solved by Euler and the BEST theorem. Leonhard Euler was the greatest mathematician of modern era, and definitely the most productive one publishing close to 900 books. Physicist Richard Feynman described Euler formula of complex numbers as “one of the most remarkable, almost astounding, formulas in all of mathematics.”
Interestingly, his productivity seem to have increased after he became blind at age 50.
Readers may enjoy ‘daily blog posts’ written by Euler, while tutoring a German princess. They are compiled in Letters of Euler on different Subjects in Natural Philosophy Addressed to a German Princess.