Also online, also free

Is Silvanus P. Thompson's hundred year old calculus textbook, Calculus Made Easy. One of the few, if not the only math textbook that is actually a pleasure to read. It was a supplementary reading in one of the game theory classes I've taken in grad school. In New York, you're exempt from sales tax when purchasing textbooks, as long as you show a valid student ID and a syllabus that has the book you want to buy on it. Whenever I'd buy textbooks, bookstore cashiers would always ask for a student ID but were never very much interested in seeing a syllabus. And for good reason, I suppose; you'd have to be at least slightly insane to be buying, say, "Econometric Analysis" unless you needed it for school. The only time I was asked for a syllabus was while buying "Calculus Made Easy" (it wasn't online yet). "You see this one," the cashier smiled "Some people actually read for fun."