Introduction to Derivative-Free Optimization, Andrew R. Conn, Katya Scheinberg, Luis N. Vicente, 2009 Vol. MPS-SIAM Series on Optimization; 8 (Society for Industrial and Applied Mathematics)DOI: 10.1137/1.9780898718768 - A comprehensive textbook providing a rigorous yet accessible treatment of DFO methods, including direct search and model-based algorithms.
A Simplex Method for Function Minimization, John A. Nelder, Roger Mead, 1965The Computer Journal, Vol. 7 (Oxford University Press)DOI: 10.1093/comjnl/7.4.308 - The original paper introducing the Nelder-Mead simplex method, a classic direct search algorithm detailed in the section.