Submodular functions: reference request
The references I use (and like) are selected chapters in Schrijver's 3-volume Combinatorial Optimization: Polyhedra and Efficiency (Springer) and Vygen's Combinatorial Optimization (Springer). There is a book devoted to submodular functions by Fujishige: Submodular Functions and Optimization, volume 58 of Annals of Discrete Mathematics, North-Holland (2nd edition from 2005).