We have had discussion on the EdCommunity portal about the plethora of excellent GIS textbooks that have appeared in the past 25 years in print. A growing number of useful texts are now appearing online that are useful to teach GIS and learn about GIS.
I use sections of Penn State Professor David DiBiase's online text, the Nature of Geographic Information: https://www.e-education.psu.edu/natureofgeoinfo/. This text, continuously updated since 1997, begins with discussions on data, information, and database management systems before moving into a discussion of GIS. I especially like the geographic questions posed in the last chapter, and the fact that you can read comments posted by students who have used this text as part of their Penn State coursework.
I also make use of De Smith, Goodchild, and Longley's Geospatial Analysis, because it fits in well with my own belief that the power of GIS lies in the analysis: http://www.spatialanalysisonline.com/. Also available as a printed book, it discusses principles and techniques for spatial analysis, and I find the search tools especially useful.
One of my favorite people here in Colorado, Dr Joe Berry, has placed his Map Analysis book online on http://www.innovativegis.com/basis/MapAnalysis/. It is a treasure of interesting topics from landscape visualization even to shoppers’ movements in stores!
Interestingly, some of the online text resources I use are not new. This makes sense when one considers that issues of scale, accuracy, representing geographic information, and some others have been a part of GIS for decades. For example, the NCGIA Core Curriculum from 2000 still contains useful nuggets: http://www.ncgia.ucsb.edu/giscc/. Even the 1990 version, which was one of the most important items on my shelf for years at the USGS, contains useful sections, and is now online: http://www.geog.ubc.ca/courses/klink/gis.notes/ncgia/toc.html.
I also make use of the map projections and other sections from Dr. Ken Foote’s Geographer's Craft project at the University of Colorado: http://www.colorado.edu/geography/gcraft/contents.html.
A dated but useful and extensive GIS book from Meaden and Kapetsky is hosted by FAO: http://www.fao.org/docrep/003/T0446E/T0446E00.htm#TOC
What online texts do you make use of when you teach GIS?