Reference books on geostatistics:

Schabenberger, O. and Gotway, C. (2005) Statistical Methods for Spatial Data Analysis

Chapman &Hall/CRC.

Peter J. Diggle, Paulo J. Ribeiro, Jr (2007) Model-based geostatistics, Springer.

Cressie, N. (1993). Statistics for Spatial Data (Revised Ed.). John Wiley & Sons, Inc.

Chiles, J. P. and Delfiner, P. (1999) Geostatistics: Modeling Spatial Uncertainty. Wiley.

Stein, M. L. (1999) Interpolation of Spatial Data: Some Theory for Kriging. Springer.

Banerjee, S, Carlin, B., and Gelfand, A. E. (2004) Hierarchical modeling and analysis for spatial data. Chapman & Hall

Wackernagel, Hans (1998) Multivariate Geostatistics (2nd ed.) Springer.

Some application-specific references:

Kitanidis, P.K. (1997) Introduction to geostatistics: applications in hydrology.

Goovaerts,Pierre (1999) Geostatistics for Natural Resource Evaluation.

Olea, R. A. (1999) Geostatistics for Engineers and Earth Scientists.

Christakos, G (2000) Modern Spatiotemporal Geostatistics.

Webster, R. and Webster, M (2001) Geostatistics for Environmental Scientists.