The First Six Books of The Elements of Euclid by Oliver Byrne – with the subtitle: In which coloured diagrams and symbols are used instead of letters for the greater ease of learners. This is a classic, illustrated textbook on geometry and a remarkable example of Victorian printing which has been described as one of the oddest and most beautiful books of the 19th century. It was published in 1847 but it still is very readable thanks to Oliver Byrnes comprehensible educational style of writing.
Download it here (306 pages/11 MB):
The work was reprinted by Taschen after 160 years. You can buy the print edition here: Link