The following references provide additional information on contact analysis with finite element methods. They allow the interested user to explore the topic in more depth.
General texts on contact analysis
Belytschko, T., W. K. Liu, and B. Moran, Nonlinear Finite Elements for Continua and Structures, Wiley & Sons, 2000.
Crisfield, M. A., Non-linear Finite Element Analysis of Solids and Structures, Volume II: Advanced Topics, Wiley & Sons, 1997.
Johnson, K. L., Contact Mechanics, Cambridge, 1985.
Oden, J. T., and G. F. Carey, Finite Elements: Special Problems in Solid Mechanics, Prentice-Hall, 1984.