A few papers related to SNOPT and the underlying algorithms are:
SNOPT users manual is the basic refrence for using the SNOPT directly. It has some theory as well. (download)
SNOPT: An SQP Algorithm For Large-Scale Constrained Optimization. (download)
A Sequential Quadratic Programming Algorithm Using An Incomplete Solution Of The Subproblem. (download)
And SNOPT v7.1 itself can be downloaded from here.
Lecture notes on nonlinear constraints
Lecture notes on optimality conditions
Numerical Solution of Saddle Point Problems
Facility location problem
Facility location models for distribution system design
Local Relaxation Methods for Nonlinear Facility Location Problems
Slides for the substation placement problem