I'm always on the look out for quality, authoritative, [and free] GIS resources. Here are a few books I've heard about recently that are well worth looking at:

Nature of Geographic Information
DiBiase, D. (2008-09). Nature of Geographic Information: An Open Geospatial Textbook. The Pennsylvania State University, College of Earth and Mineral Sciences Open Educational Resources Initiative http://natureofgeoinfo.org. David DiBiase (dibiase@psu.edu) The Pennsylvania State University Department of Geography, and director of John A. Dutton e-Education Institute, College of Earth and Mineral Sciences. Mr. DiBiase manages the Department's Certificate Program in Geographic Information Systems (GIS) and Master of GIS (MGIS) degree program, both of which are offered online through Penn State's World Campus.

With the blessings of both Penn State and ESRI, David DiBiase is sharing this text with students and teachers everywhere as part of an Open Educational Resources initiative. You are welcome to use and re-use materials that appear in this text (other than those copyrighted by others) subject to the licensing agreement linked to the bottom of every page. Additional open educational resources are available here: http://open.ems.psu.edu/courseware.

Geospatial Analysis
de Smith, Goodchild, Longley (3rd edition © 2006-2009). Geospatial Analysis - a comprehensive guide. University College London, UK Department of Civil, Environmental & Geomatic Engineering http://www.spatialanalysisonline.com.
The WEB Version of this book is FREE, and addresses the full spectrum of spatial analysis and associated modeling techniques that are provided within currently available and widely used geographic information systems (GIS) and associated software.