"The golden age" of coloring books is believed to be 1960-s, because almost all characters were depicted in them, as well as some products (like Planter Peanuts or Campbell Soup)
Today there is a large set of coloring books. You can find coloring-cursive books, books outliners, for boys (all sorts of appliances, machines, robots, etc.), for girls (fairy-tale characters, dolls, funny funny little animals). There's even a wallpaper coloring books that opens a wide field to have enough space for young artist.