This is why the "PDF search" is so popular among autodidacts. Modern books are great for looking up a formula for a test; Peterson’s book is great for sitting down on a Saturday morning with a cup of coffee to finally understand why the integral works.
In modern textbooks, diagrams often feel like decorative footnotes to the algebra. In Peterson’s era, the diagram was the explanation. Peterson was a master of "Analytic Geometry"—a discipline that has largely been subsumed into pre-calculus or eliminated entirely in some curriculums. thurman peterson calculus pdf