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.