Ask HN: Which science/math books from the masters are still good reads?

I have found some pretty good reviews from old book translations from Great mathematicians like "Elements of Algebra" by Leonard Euler or "Disquisitiones Arithmeticae" from Gauss. I wonder how many of this kind of jewels are there which are very readable and whose subject is masterfully explained which we never get to read because of newer horrible textbooks that are used in current education curricula.