I make over 1/3 of my income from stock - in 5 figures.. Yes, you have to pay attention as to what is saleable or not. It is a great way to make a living. I know many photographers who do nothing else and produce over 6 figures per year in income. I could make more, but I have my niche that I am happy with. Cards don't make a lot of money nor book covers. My sales are spread over many usages - advertising, editorial, travel, brochures, etc., that include these but are not specific to those industries. My income from stock is generated on a few hundred, not thousands of images.