100# gloss book paper is half the thickness of 100# gloss cover. Your Cover Stock paper is more comparible to a thick cardstock, while your Gloss Book Stock Papers are comparible to a magazine paper. Both can be used with a UV or AQ gloss coating.
Gloss Book papers are usually more ideal for brochures while your gloss cover paper is better suited for something like a flyer, brochures, posters or as covers to magazines or booklets.
However, if you're printing a business card or postcard I suggest using a thicker cardstock than 100# gloss cover. 14pt or 16pt cardstock, or even a 20pt plastic is more desirable.