This appendix is only an introduction to projections. For more information about map projections, technical specifics, or cartographic principles, please refer to the following online resources:
http://www.geog.ucsb.edu/~dylan/mtpe/geosphere/topics/rs/howis.html
The main PROJ.4 web site includes links to more technical documents that describe the math and parameters involved with specific projections. See the references section at the bottom of the main web page for more links at http://proj.maptools.org.
This resource includes significant information on map projections (http://atlas.gc.ca/site/english/learningresources/carto_corner/index.html).
Check here for an overview of specific projections and theory: http://erg.usgs.gov/isb/pubs/MapProjections/projections.html.
Command-line utilities and libraries allow comprehensive access to raster (GDAL) and vector (OGR) data, including projection support; check out http://gdal.org.
This site has a simple tutorial focused on understanding the trigonometric concepts behind spherical measurements and navigation; find it at http://www.dynagen.co.za/eugene/where/sphertrg.html.
Here you find many links to articles, papers and country-specific map projection information http://www.ferris.edu/htmls/academics/course.offerings/burtchr/geodesy/datums.html.