Applied Calculus (Calaway, Hoffman and Lippman)
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An openly licensed applied calculus textbook, covering derivatives, integrals, and an intro to multivariable calculus. This book is heavily remixed from Dale Hoffman's Contemporary Calculus textbook, and retains the same conceptual focus from that text.
Thumbnail: A straight line tangent to a curve can be determined from the derivative of that curve. (CC BY-SA 3.0 Unported; AxelBoldt via Wikicommons)