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Can anyone recommend a good reference or tutorial that explains the principles of spatial references, projections, and coordinate systems?
We are a small team of programmers working with GIS technology, and we often get snagged when working with data in different SRS/CS because we don't understand the concepts.
Personally, for the cliff's notes I find the ESRI ArcGIS desktop software help system useful, and and also the ESRI book Understanding Map Projections, its first 30 pages are not unlike a short textbook, followed by ~70 pages of appendix on individual projections, their uses, strengths, weaknesses, etc.
From these, you'll quickly understand all the pieces of a complete SR - spheroid + datum + meridian + unit of measure => GCS; and projection + parameters + unit of measure + GCS => PCS.
For all the math, try the classic USGS Professional Paper 1395 by John P. Synder - Map Projections - A Working Manual. You might find a PDF somewhere if you google it.
Finally, take a look at the 'documentation' links at the bottom of the page for the Proj.4 library.
And as a DB of spatial references, I tend to refer to http://www.spatialreference.org/ or another ESRI list provided as part of the docs for their ArcGIS Server REST API.
Hope that helps.
Answering a question about the difference between a projection and a datum, Bill Huber links to his article in the Directions Magazine where he gives introductory explanations of georeferencing, datums, spheroids, and the like.
As short and concise as it can be!
I think Carlos Furuti's tutorial is a really good resource.
http://www.progonos.com/furuti/MapProj/CartIndex/cartIndex.html
The PostGIS in action book has a great chapter, 6 - Spatial reference system considerations, on these subjects, although it is (of course) a bit targeted at PostGIS use. Well worth checking out.
The OGP have lots of useful information on geodesy, try:
http://www.ogp.org.uk/pubs/373-01.pdf
They also have a GIGS (Geospatial Integrity of GeoScience Software) program, more info here and also via OGP:
http://www.cain-barnes.com/index.php/geospatial-integrity-of-geoscience-software
I just ran across NOAA's "What is a Datum".
They seem to have more info on vertical datums than other sites I've seen.
(I guess data is no longer the plural form of datum.)
Here's one for Arc that goes through several scenarios using vector data: http://blog.geographyforever.org/2013/03/25/working-with-spatial-reference-systems-in-arcgis/