Pantone sells the standard, I don't realy, Basiclly they establish the standard and lience it to color media "inks, Dies, film etc..." manufactures. Inorder to use the name Pantone they must meet the established standard.
Pantone publishs the guides, so you may choose colors, from the standard. Now pantone also produces a 4-Color process guide, but this is depandent on the printer using pantone process inks. But considering the multiple variables involved in process printing, the guide is only a close approxmation.
As Hyperbol points out, cost is an important to factor, spot ink are expensive, as most have to be mixed, and the press must go through a wash up inorder to change inks. In General spot color is only cost effective if runing less then 3 colors.
But cost is often varied, it's best to consult with your printer, he may have a press already running, 4-color, and might perfer to offer a deal over, taking dont the press washing it up. A the next job down the line might be 4-color again.
If you design requires over 3 colors I would advise, thinking carefull before choosing a spot ink. The exception where critical element exists such as a estabished product color, or logo (Coke red, etc...), a large solid area of color is required, or color exist that are unreproducable by any other means. For instance Dayglows, Metalics, Pearls, and Varnishs.
I think the "C" you are refering to is the Pantone paper stock code, as in Pantone 108c, or Pantone 108u etc... This codes specific the stock the guide was printed on as the resulting color will change depending on the paper type. Typically guides are printed on two stocks "C" Coated and "U" Uncoated.
A good resource is: www.pantone.com
The Printing Pocket Pal is top on my list as well, I have a few others I use in classes, but I will have to get you the names from home later. Thou some you may have to find used at they are likely out of print.
A few other sites that are useful:
http://www.dtp-aus.com/dtpstrt.html
www.creativepro.com
J. Stuart J.
(Edited by jstuartj on 10-14-2004 21:12)