1-4 of 4 Books

The best approximation method
By Theodore V. Hromadka,Chung-Cheng Yen,George F. Pinder

Foundations of quantization for probability distributions
By Siegfried Graf,Harald Luschgy

N-widths in approximation theory
By Allan Pinkus

Spaces of approximating functions with Haar-like conditions
By Kazuaki Kitahara