Felix Klein’s Ueber Riemann’s Theorie der Algebraischen Functionen (1882) is one of the celebrated documents of mathematical intuition: Riemann’s theory of algebraic functions and their integrals, presented not through rigorous machinery but through flows of electricity on surfaces — the physical picture Klein believed lay behind Riemann’s own thinking.
Mathematicians still argue about how Riemann thought; Klein’s little book is the classic case for the intuitive answer. Download the free EPUB, in the original German, for function theory as pure imagination.