Notebook of Leonardo da Vinci, The Codex Arundel π
A collection of papers written in Italian by Leonardo da Vinci (b. 1452, d. 1519), in his characteristic left-handed mirror-writing (reading from right to left), including diagrams, drawings and brief texts, covering a broad range of topics in science and art, as well as personal notes. It's available online for free at the British Library online portal.