"Methods of Modern Mathematical Physics" (by Arnaud S. P.); "Perturbation Methods" (by J. R. L. McLachlan)

"Methods of Modern Mathematical Physics" by Arnaud S. P. explores advanced mathematical techniques used to analyze physical systems, such as quantum mechanics and relativity, emphasizing rigorous frameworks and modern approaches. "Perturbation Methods" by J. R. L. McLachlan focuses on techniques for solving complex problems where exact solutions are difficult; it involves gradually adjusting a known solution to approximate solutions of more complex problems. Both works provide systematic, analytical tools for physicists and mathematicians to understand, approximate, and predict behaviors of physical systems across various fields.