Calculus: Early Transcendentals, James Stewart, 2015 (Cengage Learning) - A widely used textbook for multivariable calculus, offering a complete treatment of directional derivatives, gradients, and their geometric interpretations.
Multivariable Calculus (18.02SC), Denis Auroux, 2010 (MIT OpenCourseWare) - An online course from MIT covering multivariable calculus concepts, including lectures and problem sets on directional derivatives and gradients.
Mathematics for Machine Learning, Marc Peter Deisenroth, A. Aldo Faisal, and Cheng Soon Ong, 2020 (Cambridge University Press) - This textbook provides the mathematical foundations for machine learning, with a section dedicated to calculus essential for understanding gradient-based optimization.