Calculus: Early Transcendentals, James Stewart, 2016 (Cengage Learning) - Standard undergraduate textbook for multivariable calculus, providing clear explanations and examples of functions of multiple variables and their visualization.
Mathematics for Machine Learning, Marc Peter Deisenroth, A. Aldo Faisal, Cheng Soon Ong, 2020 (Cambridge University Press)DOI: 10.1017/9781108679930 - Covers the mathematical foundations of machine learning, including multivariable functions and their relevance to cost functions and model parameters.
Multivariable Calculus (18.02SC), Denis Auroux, Arthur Mattuck, Jeremy Orloff, John Lewis, Heidi Burgiel, Christine Breiner, David Jordan, Joel Lewis, 2010 (MIT OpenCourseWare) - A complete course with video lectures, lecture notes, and practice problems on multivariable calculus, including functions of several variables and their geometry.