These are some books we recommend to support your knowledge of GIS.

Beyond Mapping:  concepts, algorithms, and issues in GIS, by Joseph K. Berry.  GIS World, Inc, 1993.

Cartographies of Danger:  mapping hazards in America, by Mark Monmonier.  University of Chicago Press, 1997.

Fundamentals of Geographic Information Systems, by Michael N. DeMers.  John Wiley & Sons, Inc, 1997.

Getting to Know ArcGIS Desktop: The basics of ArcView, ArcEditory, and ArcInfo, updated for ArcGIS 9, by Tim Ormsby, Eileen Napoleaon, Robert Burke, Carolyn Groessl, Laura Feaster.  ESRI Press, 2004.

How to Lie with Maps, by Mark Monmonier and H. J. De Blij.  Second edition, University of Chicago Press, 1996.

Map Use and Analysis, by John Campbell.  Second edition, Wm. C. Brown Publishers, 1993.

Principles of Geographic Information Systems, by Peter A. Burrough and Rachael A. McDonnell.  Oxford University Press, 1998. 

Spatial Reasoning for Effective GIS, by Joseph K. Berry.  GIS World, Inc, 1995.  Includes a collection of mathematical formulae by Nigel Waters.