"Numerical Linear Algebra" by Trefethen and Bau. "Advanced Mathematical Methods for Scientists and Engineers: Asymptotic Methods and Perturbation Theory" by Be

"Numerical Linear Algebra" by Trefethen and Bau. "Advanced Mathematical Methods for Scientists and Engineers: Asymptotic Methods and Perturbation Theory" by Bender and Orszag. I'm not sure if they meet your definition of being well-written exactly as you say above, but these are extremely well-written math books. This isn't a math book, but I was also a big fan of "Vector Quantization and Signal Compression" by Gersho and Gray.