I came across this quote in a 1947 paper by Carl Boyer: "[Euler's 1748 book] The Introductio is referred to frequently by historians, but its significance is generally underestimated. This book, it seems to me, is the most influential textbook of modern times. It is the work which made the function concept basic in mathematics. It popularized the definition of logarithms as exponents and the definitions of the trigonometric functions as ratios. It crystallized the distinctions between algebraic and transcendental functions and between elementary and higher functions. It developed the use of polar coordinates and of the parametric representation of curves. Many of our commonplace notations are derived from it. In a word, the Introductio did for elementary analysis what the Elements did for geometry. It is, moreover, one of the earliest textbooks on college-level mathematics which a modern student can study with ease and enjoyment, with few of the anachronisms which perplex and annoy the reader of many a classical treatise."
Below I have a link to an old blog post I wrote about one neat paragraph in the Introductio.