Gottlob Frege’s Die Grundlagen der Arithmetik (1884) asked a question philosophy had grown too comfortable to press — what, exactly, is a number? — and answered it with a rigour that created modern philosophy of mathematics. Demolishing empiricist and psychologistic accounts in some of the sharpest polemic ever written by a logician, Frege argues that arithmetic is logic, and numbers are objects given through it.
Russell built on it, Wittgenstein revered it, and analytic philosophy descends from its pages. Download the free EPUB and read the little book that redirected logic — in the original German, exactly as Frege wrote it.