Disquisitiones Arithmeticae (1801)

Carl Friedrich Gauss's own book, written in Latin and published in 1801 at the age of twenty-four, unifying earlier scattered results on congruences and quadratic forms from Fermat, Euler, Lagrange and Legendre into one systematic framework across seven sections, with original contributions including quadratic reciprocity and the construction of a regular seventeen-sided polygon by ruler and compass. It transformed number theory from a collection of isolated results into a coherent discipline built on rigorous proof and is regarded as one of the most influential mathematics books ever written.