Euclid‘s Elements is the most successful and influential textbook ever written. Only the Bible has been published in greater number. It is a beautiful illustration of an axiomatic approach to maths which is still the basis of our maths today. Many high-school students learn but a fraction of the postulates in Elements throughout their whole school career and often no geometry that isn’t included in this 2300 year old book.
Amazingly a Professor of Mathematics at Clark University has translated all thirteen books of Elements and made them available online. This would be fantastic in itself but the icing on the cake is that there is an interactive geometry applet to support each postulate so you can literally interact with the maths!