Approximation Theory Using Positive Linear Operators by Radu Paltanea
Offers an examination of the multivariate approximation case
Special focus on the Bernstein operators, including applications, and on two new classes of Bernstein-type operators
Many general estimates, leaving room for future applications (e.g. the B-spline case)
Extensions to approximation operators acting on spaces of vector functions
Historical perspective in the form of previous significant results