You may be aware that I am a fan of Catharina Klein; collecting her postcards, book illustrations and more recently one or two of her painting books. I would dearly love to own an original painting but have never seen one, signed by her, available. I have one that looks very much like her work but it is unsigned, so is believed to be by one of her followers. She must have painted thousands of pictures during her long painting career and always signed her work, so it is believed.
Catharina had a studio in Berlin from which she held art classes but this was not her only method of helping others to paint. In the early 1900s Raphael Tuck and Sons published a number of her painting books. These had either two or four of her postcards on the left hand page with outlines of the cards on the right hand side which could be coloured. Similar to the adult painting books that have become popular today. The paintings could be removed from the book and sent as postcards.
I am not quite sure how many books were published but I have found three and I expect there are more out there.